There are lots of good places on the Web (and vendors on this sight). It depends on what your looking for. I look for quality first then price (including shipping). Its hard to be assured of quality, so you end up finding one vendor that has provided good parts in the past and favoring them for the next purchase. But no one has everything, and things like ease of returns, or talking to someone knowledgeable varies widely, and what vendor is best for you will vary based on how you value these things.
For overall price, shipping, quality of the parts I have gotten, and website to find what your looking for I usually start with Classic Parts:
http://www.classicparts.com
But there are lots of good vendors I have bought parts from and been pleased with their products:
Classic Industries:
http://www.classicindustries.com/shop/truck/parts/
GMCPauls:
http://www.gmcpaulstruckparts.com/index.htm
Classic Heartbeat:
http://www.classicheartbeat.com/products1.htm
Southern Kentucky Classics:
http://sokyclassics.com/
Summit Racing:
https://www.summitracing.com/ (free shipping over $100)
I have bought lots of parts new and used from the vendors here, there is no better place to get that hard to find one-of-a-kind part that no one re-manufactures.
LMC has lots of diagrams, but as mentioned above the quality and authenticity of their parts (e.g. bolts with the wrong "head") is spotty. I could give several examples, but I still order from them when the price/shipping costs, and possibility of getting the wrong part is in their favor.
The big killer for me is the various ways shipping is calculated, and what determines where I buy. Most have a fixed price based to the $$ value of the sale, not weight or size. So when I need a part, I try and plan ahead and combine into one purchase, and size the order to not kick me up the the next higher price shipping. ($9.95 shipping for 3 #10 screws that come in a 1st class envelope ticks me off). But then you end of searching around to find where you can get that combination of parts for the best price, including shipping, and you end up buying from different vendors even if you have one you prefer.
Hope this helps, sorry for any good vendors I didn't mention, there are allot selling parts for our trucks. Good luck and happy parts hunting. - Ken