These guys say they have it in stock, https://gun-ammos.com/product/smith-wesson-mp15-sport-ii-5-56mm-ar-15-with-betsy-ross-flag-reciever/
And these guys say contact us for availability
So they'd be a maybe
herdtacticalshop.com/rifles/semi-automatic-rifles/smith-wesson-mp-15-sport-ii-betsy-ross-flag-30rd-magazine-2851414
I'd go ahead and sign up for the waiting list at buds too incase you can't secure it anywhere else.
budsgunshop.com/mobile/product/121908/redirect

Good Luck. The pre owned or secondary market may be your best bet.

https://libertytreeguns.com/product/sw-mp15-sptii-brf-556-16-30rd-blk

Geoffrey, this is about as rare as hens teeth Sir. I would check on large gun and accessory sites like (6)un (8)roker, Armslist etc as discontinued and hard to find firearms and accessories show up for sale by private individuals and small shops. Just replace (6) & (8) with G & B.