You might want to have a look in the Clubhouse forum:
theclubhouse1.net/.../index.php
It's a modeler's forum, but heavy on fantasy and sci-fi figure modeling.
An online shop, with an attendant forum, is e2046.com:
www.e2046.com/index.php
They have a huge selection of subjects, from sci-fi and fantasy to anime. They have a lot of recast items, too, but a lot of their own original products. Their forum is at
http://forum.e2046.com/index.php
I'd look on eBay, too. I build Maschinen Krieger kits--you might want to consider a powered combat suit, too--and I have searches for 1/20 figures, and the various makers of figures and accessories. The figures themselves don't have that much of a sci-fi look to them, though. The genre is really "military sci-fi", and so, the opponents look like Space *** or British 8th Army, generally speaking. They're a lot of fun to build, though. There is also a huge selection of auctions for figure kits at any given time on eBay; my search for "character kits" returns about 7600 individual auctions.
You might also want to take a look at HobbLink Japan (www.hlj.com) to see things that aren't generally available on this side of the Pacific
Hope that all helps!