ahh, my hours and days and weeks and months of researching just this sort of thing has finally come to a head!

here's a few links to get you started heading in the right direction. The Expensive stuff first:

Patriot Ordnance Factory, High end ARs and parts

Troy Industries: Excellent repuatation for quality solid equipment.

Brownells: If they don't have the part you're looking for on an AR, then you probably don't need it.

GG&G: haven't used them per...d things about thier aftermarket sights.

alot of the custom manufacturers, and even DPMS, will carry different options for sighting equipment as well.

Less expensive options next:

one of the many cheaper options

and the cheapest option I know of, but you may want to check a couple vendors for pricing, might get lucky and shave a few bucks. Magpul MBUS polymer sight system