The AR-15 Bolt Cam Pin is part of the bolt carrier. This pin goes through the carrier and through a hole in the bolt. The firing pin holds this in place as it has a hole in the middle of it that the firing pin goes through.
This page shows how to assemble an AR-15 bolt and carrier and it shows where the cam pin goes.
Go here to look at prices to purchase a cam pin, remember that usually this part comes with a bolt and carrier group.