A longer version of this post originally appeared in 2010, and was reposted in 2017 with updates, but the question keeps cropping up in various forums so here’s another update. Many links in the previous post are broken now. 
A lot of miniatures people turn their noses up at old Grenadier and Heritage and Minifigs figures. I will grant that many modern figures, which take advantage of sculpting and molding techniques unavailable to the original manufacturers (and an aesthetic sharpened by the intervening years of fantasy illustration, comics, etc.) are often quite impressive. The level of animation, and the overall quality are amazing. The crisp detail, and the fact the pieces fit perfectly make them a joy to assemble and paint. But I still love the old school minis too. They often have a gritty realism modern figures just lack, and an undefinable sense of character.
Heritage Models has a site devoted to the Dungeon Dwellers line, Dungeon Dwellers info. If you didn’t know, this line would have been their “Dungeons & Dragons” line, but the license agreement never got signed and instead Grenadier would get the license for AD&D miniatures. There were several Yahoo groups devoted to collecting particular figure lines and they all had some of the original sculptors, mold-makers, or casters present to share memories. The activity slowly shifted to other forums and you might still be able to find groups on Facebook, blogs, etc., but these are all too ephemeral to link any more.
I love and hate Games Workshop/Citadel figures. They are certainly nice looking. The only things not to like are the scale creep and cost. Citadel minis, back in the late 1980s, were the first figures I had that just didn’t quite fit with my Grenadier, Ral Partha, and Heritage stuff. Ral Partha was always a slight bit smaller than the others, but with Citadel I could tell the scale was actually shifting. Of course nowadays, almost all modern figures are a little bigger than before. “28mm”, “30mm”, “heroic 28mm scale”, even “32mm” are bandied about, whereas in the olden days everyone claimed to be making 25mm figures, whether that 25mm was toes-to-eyes, toes-to-tip of head, or just scale of 1/72 (25mm=6′). Confusingly, 1/72 is sometimes referred to as 20mm scale, since most humans were under 6′ in the historical periods they model. Your vintage Ral Prtha might be close to 20mm, while Archive or Grenadier were more like 25mm+. Some of my newer Reaper and Kenzer Co. figures absolutely tower over my old figures. And that is too bad, because no-one chooses a Ral Partha figure any more for their PC in the games I’ve been playing. They just look too runty. In fact we’ve been using a Ral Partha mounted fighter as a Dwarf on a pony, and I’m probably the only one who realizes the figure was “meant” to be a human! Still, when Citadel was making RPG minis instead of exclusively Warhammer/Warhammer 40k/other branded IP minis, they made some seriously awesome figures. And they made so many that there is a whole wiki just for Citadel, which rivals the Lost Minis Wiki! But readers of this blog may be more interested in another site that just focuses on Citadel’s old AD&D/D&D lines.
The Lost Minis Wiki was created in 2009 with the explicit intention of covering all the out-of-production lines and models, and I can kill hours there. Update: The wiki is now also awash in newer and current lines. Mission creep, I guess. But you can still find lots of old stuff. The Lost Minis Wiki has vast amounts of unpainted lead, but we really want to see the painted stuff, right?
Stuff of Legends hasn’t been updated much lately, but as far as I know it was the first site devoted to classic minis. There is also a site devoted just to dwarves.
Anyway I found a legal copy of the Armory’s Buying Guide to Fantasy Miniatures at the Mega Minis Magazine site. There is a stunning array of old catalogs there to drool over, with images of miniatures that you can only hope to scrounge up at a convention or eBay. But if you love classic minis, the good news is that there are both new lines that are inspired by older lines, and a few companies still casting the classic figures. Update: although Mega Minis is out of business, the first link still works. The second is now a link to the Wayback Machine’s backup.
You can find many old miniatures for sale second hand in the usual places buy things second hand, like eBay, Craigslist, and similar, or sometimes hobby shops and thrift stores. (Last week I found some in a Half Price Books store, of all places.) But some you can still buy brand spanking new, often in better metal alloys than the originals. Here are some options:
Classic Miniatures is recasting many Heritage models, as well as some from other defunct companies. It’s a more of a hobby than a business, though, so please be patient if you place orders.
“Minifigs” today is usually taken to mean Lego people, but the original Minifigs company is putting their old fantasy lines back in production. Details here.
Ironwind Metals, which rose from the ashes of Ral Partha, is producing some of the old RP lines, and Kickstarting more. See the details here.
Thunderbolt Mountain, Tom Meier’s company, is producing figures similar to his Ral Partha classics, but in a more “modern” 28mm scale. Update: new site launched in 2018, but no updates since…
McEwan Miniatures, some of which were sold as part of the Masterpiece Miniatures line, are still being produced in part here: McEwan Miniatures.
Mirliton, an Italian company, is producing some of the latest Grenadier lines, including some of the old Wizzards & Warriors/AD&D lines! Pricey but classics.
Mega Minis produces original figures as well as an extensive array of older lines. They are providing a great service but I wish they didn’t cancel lines after short runs. Update: Mega Minis, sadly, is out of business. Their molds may have been picked up by other companies. Their original stuff is now at Johnnyborg Castings. These seem to be Kick Starters so caveat emptor.
Viking Forge is producing classic Asgard minis … the ones illustrated in the Armory ads in old Dragon Magazines!
Armorcast is producing many old Lance& Laser/Castle Creations figures, as well as new designs in the old school aesthetic.
If you are looking for old Citadel, there have been occasional revivals of some models, but a consistent source is Wargames Foundry, which has some of the Citadel dark ages vikings and Normans here.
A few other companies are also still producing older lines, such as RAFM and Essex.
And others are producing new lines with old school aesthetics.
You’d have to be living under a rock not to know about Otherworld Miniatures, which is creating minis directly inspired by the classic illustrations of Sutherland, Trampier, etc. Update: But they are in 28mm scale, not classic 25mm.
Pacesetter Games is producing some old-school designs originally created for a disastrous KickStarter by another company. They look nice though.
I have some hopes for Satanic Panic which is doing some old style “gnolls” after the manner of early 1980s Citadel “Fantasy Tribe Gnolls”.
Skull & Crown is doing a line of skeletons that follow the aesthetic of Minifig’s Valley of the Four Winds undead, themselves based on Brughel’s Triumph of Death.
No doubt there are more… maybe another update in a few years…
