If you haven't already done so, I'd also recommend you check out AR15.com. Its pretty much the best community devoted to ARs on the planet. There are lots of stickied info to be found.
Again, if you haven't already done so, check with AR15.com, decide what you want or "need" & then check back with the businesses Scotty listed above to source the parts/accessories. I've dealt with them all and you can't go wrong with any of them. However, it also pays to keep an eye on the EE as well.