lmctruck.com is REALLY good for referncing/finding parts, but a lot of people on here dislike their pricing and sometimes quality. I order things I dont care much about there (headlight wiring, mirrors, etc)
www.gmcpauls.com is where I got my fuel tank kit. Most people on here recommend using the board vendors as much as possible, and he is one of them.
For suspension, everyone said Early classic enterprises (
http://www.earlyclassic.com/) and I went there and do NOT regret it one bit.
Basically, pick what you are working on, search on here, find vendors that sell the parts, and review other people's installs and reviews and form an opinion.