You can order the BCG and some other parts via smartgunner.com.
There is high demand right now for complete rifles and complete halves, so DD and some other manufacturers seem to be temporarily halting sales of the lower profit but scarce items like stripped receivers. I'm sure those sales will restart when the current rush dissipates, but whether that's one week or a year I don't know. DD makes high quality stuff, but there are many other options for the few parts like stripped receivers that they are not currently selling.
BTW, smartgunner has prices on complete uppers that may make you reconsider building a stripped one. If you do want a stripped one, BCM was in stock as of yesterday with stripped uppers, both fully stripped and with just the dust cover and FA installed.