Making a scuba adaptor?

Is there a way to DIY the adaptor/connections at a hardware store/in town to setup a scuba tank for filling my air gun?

when i bought my gun the guy was really trying to get me to buy a hill pump, but i was a bit wary of the moisture pumps put in the guns, i know the hill takes out 90% of the humidty but 10% over a period of time, seems like that would still cause some problems.

So im pretty sure i can get a used tank locally for pretty cheap, but i won’t know what kind of fitting (k valve or din), until i get it. And id prefer to not put in another internet order as i would have to pay shipping fee’s. So surely i should be able to build the fittings and stuff to air my gun up right? between a scuba/dive shop and a hardware store, there should be a way? anyone done this?

$100 for an adaptor sounds rediculess, $30 for a foot of hose is crazy, there must be a way to get this stuff at a reasonable price