Where have you been finding 6.8 BCGs?
You will get a lot more expertise and experience with the 6.8 on 6.8 forums:
http://68forums.com/forums/forum.php?
Off the top of my head, you can get BCGs from a lot of manufacturers for the AR platform. Noveske, Daniel Defense, Lewis Machine and Tool, Wilson Combat, LWRC,…etc.
You are best to grab a complete 5.56 BCG and swap in a 6.8 bolt. A bit more expensive but complete 6.8 BCG’s are very hard to find right now. PSA has some 6.8 bolts available today (first time in a while) and you shouldn’t have too much trouble finding a 5.56 BCG. Good luck.
I would suggest looking into the friendly back-order policies at MidwayUSA and DSG Arms, which carry Daniel Defense and LMT BCGs respectively.
If you happen to find just a bolt somewhere, BCM sells their bolt carriers as a separate piece (no logo but the same quality) and they are sometimes in stock even when the BCM complete BCGs are out of stock (though I think those were in stock a few days ago anyway). AFAIK BCM and Colt don’t make 6.8 SPC bolts.
If I were building a 6.8 I would probably be using an AR Performance barrel and get one of their “superbolts,” or just buy the whole upper from them as a unit.
You won’t be able to get a BCM carrier, I have checked 4-5 times per week for the last 6 months and they haven’t been in stock. Their full BCG’s are generally always in stock (small exceptions of a few days from time to time). ARP has also had spotty inventory at best and I finally had to order a Stag(CMT) 6.8 bolt and grab a complete BCM BCG and swap the bolt. In my oppinion if you want a quality 6.8 BCG this year, you will have to buy the components separtely. Good luck.