I've bought a couple different AR-15 parts kits from M&A Parts
http://www.mapartsinc.com and have been very satisfied. Not to say there were absolutely no wrinkles, but nothing to get excited over. They're out of IL and usually have a few tables at the funshow at the DeltaPlex. There are quite a few good suppliers, but these guys have the right price and are more likely than many others to have what you're looking for in stock. They're generally well regarded over at AR15.com ("arfcom"), which is THE definitive site to check out if you are interested at all in building, owning, or shooting an AR or one of its derivatives.
Attached is a target I shot with my first varmint kit I bought from them. (This was also the first AR kit I built -- it wasn't difficult, but it gets easier every time and no specialized tools are required when you buy kits like these guys sell.) As you can see, it's 5 shots into less than 2" at 200yds [nothing special for a benchrest competitor, but I'm proud of it]. The recipe is 55gr MidwayUSA Dogtown bullets over 25.3gr of Hodgdon Benchmark. Primers were CCI small rifle, cases were Malaysian 5.56 surplus from the early eighties. Loaded on my Lee turret press.
If you have any questions, feel free to post here.
