Depends on what you're looking for, naturally there's the 100$ import restriction rule... a lot of times your standard issued M4 stock is good enough, anything else is a lot more personalization.
Questar, OST, Wolverine, and Fabsports are my go to guys for AR accessories... ATRS has lots of stuff too, but they tend to be a little more pricey than other guys... if you can't find what you want, Brownells does ship to the states, and they have a large selection, but only a small subset of their inventory is available for international trade.
That being said.... what are you looking for? Magpul's really hot right now, and their MOE line of accessories is cost effective, (by no means can you use the term 'cheap' for Magpul)... VLTOR is also another great company that breates accessories for the AR platform, I think Armtac is the primary dealer for them, though I've never dealt with them before.