Dean, unfortunately due to ultra high demand, online retailers are sold out. 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.

That's likely to be a hard find even without the crazy panic buying going on right now I'd say your best bet is hit the "notify me when available" button then act fast when you get that notification.
Or if there's an Academy sports near you they're sight says its available to pick up in store.
Other then that keep an eye on the auction sights, good luck stay safe.