In the last few years, 3D printing has come a very long way -- gone are the days where every printing step is painfully visible, and highly-detailed resins plus the availability of free modeling software like Blender are allowing 3D modelers (who may not have the slightest skill with green stuff) to print off some incredible bits, minis, and terrain (sometimes even as sprues, which is hilarious to me since they don't start out as an injection mold).
Shapeways is a 3D printing site, allowing its users to upload 3D models that they will then print to order. Over the last year (I don't have proof, but a year ago I was looking for the same thing and found significantly less than I did this year) the custom bits market on Shapeways has blown up, where you can find just about anything.
Looking for Rhino doors for your Ork Lootas? Shoulder pads to make your vanilla marines stand out? A Fleur de Lis dozer blade for your Immolator? A dragon/demon/Tyranid model that none of your friends have EVER seen (see the picture)? Throw 28mm in Shapeways' search box and go to town!
The only real downside I've found to Shapeways -- besides the prices, which are almost GW in nature -- is that most artists only carry one or two things that are applicable to any given hobby. Sure, there are exceptions, but you'll have to actually spend time looking.
But, in case you don't feel like sifting through all of that, here are some excellent links for you:
- Admiral Duck Sauce - Alternative SM models, a couple neat head sprues
- DW Toys - Backpacks, weapons, stuff for your DIY terrain to look a little better.
- BlackBits - Dreadnought weapons and top hats (for your fancy IG, I guess).
- Custom Minis - Shoulder pads galore. A few weapons, a few heads, and some other random bits, but this is THE place to find shoulder pads.
- Wolfhound Jack Designs - Dozer blades for SoB, IG, and SM.
- Vandil - Ork parts for Imperial vehicles, among other things.
- Dynath - Mostly weapon sprues, but some interesting mech bits are in there, too.
- GangWar Miniatures - Some great terrain pieces, as well as some female minis that would work well as Guard.
- ArtisticFiend - A single sprue of Marine stuff and a couple Storm Shields.
- Random Bits - Not so random as it looks like some interesting Necron alternatives, and some weapons.
- Custom Bitz - The demon/dragon/Nid posted above and some good looking wings.
- Saberwar 52nd Century - Chaos flamers, backpacks, and a pair of heads that will keep your bare-headed marines from looking like white supremacists.
Lemme know if you end up buying anything -- I would love to see how it turns out (And I'll let you know when I buy the dozer blade and probably the dragon thing).