I haven't seen spare BCGs in Boise or online for quite a while. For a while the boutique units were in stock if you wanted to pay for them but I suspect they're hard to find too these days. There are a lot of builds out there waiting for a BCG!
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek