Bolt Carrier / Control Groups (BCG) are available on AR15.com and maybe other sites. Problem is a Bravo Company BCG is going for between $300 - 500 right now. The Bravo Company price in normal times was $189 and has crept up to $225. You may find another brand for less, but Bravo's is highly thought of.
So, it's a question of how urgently do you want to finish your build ? If your answer is "I want it yesterday", then go to AR15.com and check their Equipment Exchange.
Gene in Illinois