I have been happy with the normal M-4 rear sight. They are inexpensive too if you find one pulled off someone's rifle.
as for the BUIS, common ones are:
Plastic Magpul BUIS: Magpul BUIS Rear - Black [MAG246BLK] - $54.95 : US Armorment, The Art & Science of Shooting. Shooting Supplies, Ammunition, Rifle Scopes, Gun Parts & Accessories
Troy BUIS: Troy SKD Folding Battle Sight- Rear, FBS-RA1.SKD
I still haven't found a good AR-15 optic that is sturdy, accurate, and cheap so I am all ears to what others suggest.