I would PM him and see. I've gotten several things from Owen and he's always good to deal with. You can also make a post to Trade bolt heads on the PX. I'd be willing to bet that someone here has a standard one they'd sell or trade.
Generally I try not to order anything from these guys (they once charged me ~$60 for placing an order and canceling it 5mins later), but they have one in-stock:
Thanks everyone. Ended up grabbing one from FrantRangePrecision. Great price and excellent service. Just waiting on arrival at my FFL! Just got confirmation today that my PBB prefit is almost finished. Can’t wait to put it all together.