Two good ones that have a lot of stuff...
MAS Defense
AIM Surplus
I recently purchased a new [12"]
free float handguard from MAS (along with a steel gas block and melonite nitride gas tube, that came assembled) for my 300 Blackout rifle build. I also purchased the BCG (also melonite nitride) from AIM Surplus for $99 [delivered]. Both companies have EXCELLENT customer service/support. Plus, the inventory on MAS Defense's site is live. Not XX minutes behind.
There are plenty of other stores to order from, as already mentioned. Brownell's and Midway USA are two of the larger ones. Most are solid, but there are a few to avoid. Places like Ice Arms for one (avoid all their products if you value your sanity).
For sights, it's hard to be MagPul, especially for the cost. If you want a dot/holo type sight, then you have several options (depending on your budget).
What is the maximum range you want to shoot the rifle? If you're looking for a scope, then get the best you can afford. I went with a NCstar scope for my first 5.56 AR15 (3-9x). I wish I had bought a better one (it was in the $100 range), with more magnification. It's ok out to 100 yards, but hard for me to see the holes at that distance with it.