Gunbroker is the best option. Occasionally you'll find auctions from gun busters. They will put up an entire 556 minus the regulated upper receiver area.
They aren't cheap but you get every part as a spare and you could sell off some of the things you don't need to offset the cost.
Sent from my SM-S908U using Tapatalk
They aren't cheap but you get every part as a spare and you could sell off some of the things you don't need to offset the cost.
Sent from my SM-S908U using Tapatalk