Here is are some that I have found, and I have used before (some I have not used, but have heard great things about)
Detroit Muscle for interior gaskets and whatnot. I have gotten the paint gasket set and glovebox liner, and they are super high quality:
http://www.detroitmuscletechnologies.com/
Classic Industries has a pretty good selection of interior parts, but I have never ordered from them:
http://www.classicindustries.com
Legendary also makes nice stuff, but I have never ordered from them either
http://www.legendaryautointeriors.com
Stock Interiors seems like they have a pretty good selection too, and I think they vend Trim Parts for carpet, as the instructions on their website is identical to what was in my Trim Parts carpet box:
http://www.stockinteriors.com
ANT Motorsports offers fastener kits - I have a set on order for external trim, so will post what I think about them in my build thread. No website, but they sell through Ebag:
http://stores.ebay.com/ANT-Motorsports?_rdc=1
And of course, the above mentioned Summit Racing and of course, here in the for sale ads. The FABO guys are great, I have picked up some pretty good stuff, and it is always nice to help the other guys out in funding their projects. Summit is great, but does not offer a lot for interior parts.
I am using a local upholstery guy for the headliner and seat recovering, as I have a 4 door and the vendors I have found for 4 door upholstery covers were a bit spendy.
Just my 3 cents