most of the time they just drop right in as AR-15s are supposed to be a built to a standard head-space. it is a good thing to double check anyway.
DS Arms has some good prices on the semi exotic nitride finished uppers --i think there about $275 last time i looked, their chrome lined 1-7" was $300 then.
evil black rifle girls
did have a sale on bolt complete bolt carriers. seems like the ar-15 was $95 and the m-16 carrier was $105. i bought one of there m16 bolt carrier groups and the thing was colt --not bad for $105.
e-sarco has some good prices on their bolt carriers and bolt carrier assembly. though i haven't used a sarco bolt carrier so i can't comment on the quality or lack there of. if they are surplus USGI they should be good.
there should be plenty to choose from--just shop around and you will turn up a deal.