Ebay has them in multiple sizes and quantities.
https://www.ebay.com/itm/181862013967?var=480902126814&hash=item2a57d21c0f:gUsAAOSwLqFV7trI
Mac1 hasn't had screws cups in stock for a year and a half or more. AoA has their version but while brass, not like the Mac1 versions, as they have no sleeve on the backside. Can't remember where I read it so someone with an Air Arms will have to chime in here but aren't the screw holes in the stock beveled inside?? Might make using a set of either a bit difficult if they are I'd think.