A good start is the list of CC Advertisers that should be down on the bottom left of your screen on this site. Then go to CobraCountry, if you haven't been there already, and get Curt's guide as it has a pretty complete list and commentary on each. I haven't seen anything definitive that will include all the vendors out there as they seem to come and go more frequently than anyone would like. Here's a quick list based on what I've heard and a quick look at Curt Scott's guide. IMO the most common names you see and hear are the ones with * after the name:
Kirkham*
Shelby American*
Superformance*
ERA*
Unique*
Factory Five*
Contemporary* - no longer in business, but many out there
Shell Valley / MidStates*
RUCC
Venom
B&B
Backdraft
Everett-Morrison
Hurricane
Lone Star
Shamrock
Pacific Roadsters
JBL
Emerson Motorsports
Classic Roadsters
Antique & Collectibles
West Coast Performance
When I did my search I found that I basically narrowed my list to 3-4 vendors based on 1-Price (that put Kirkham and Shelby out for me) 2 - Original dimensions, engine (wanted a 427so) and look (that put Factory Five and several of the stretched versions out) 3- Reputation / perceived quality (that put several more at the bottom of the list out). I ended up focusing on ERA, Superformance and Contemporary. To be fair, I also didn't look at the entire list above so don't know anything about many of them. Couple of other opinions:
- SPFs you really don't build - they are a complete roller and basically a modern production line car underneath.
- ERA, Unique and Factory Five seem to have the best customer/builder support (however I didn't build mine so this is pure perception)
- Kirkhams and Shelby's are top of the line and there is a notable price gap between them and the others
I have probably now exhausted my opinons, and as such will now revert to my original recommendation - see them, touch them, drive them first!