There are a range of cheap compressors that will fill / resupply any gun tank just fine, these can be both 12 V or mains powered. But thats also about as much as i know on this topic as i have not studied it much yet.
Alternative you can buy a diving air tank, and have it refilled at your local dive store, this is practical if you have such a store near buy, if you live in the middle of the country such store might be few and far between.
I got my tank at the local store, and got a 10 turn card for refilling of my own bottle, or renting one similar to it. There are plenty of shots in a 10 - 12 L bottle in you are shooting one of the smaller normal calibers, the bigger caliber guns of course burn thru air fast as they need a lot of it to push those big pellets / slugs, but .177 - .22 and .25 you have plenty of joy in a dive bottle.
I would personally go for a 300 bar bottle, even if your gun do not support that high pressure, just be mindful on your guns pressure gauge when filling it up, the additional pressure in the 300 bar big tank just mean more shots at lesser pressure.
Just be sure your local store can do those kind of pressure refill.
Steel tanks are cheaper, and just fine if you are fit and dont have to carry the tank a long way to where you shoot, if you do a cart of some sort for your gear are fine. Otherwise there are ALU tanks and Carbon fiber tanks.
Here in DK steel tanks have to be pressure tested every 10 years, alu and carbon tanks i think are every 5 years,,,,, i am personally contemplating getting a slightly smaller but higher pressure carbon bottle as i am old and declining in health, and the steel tank are getting heavy carrying it down from my #2 floor apartment.
The scuba store probably can not be bothered with a small gun air tank, but a big dive bottle they can do, and of course you dont have to get a humongous bottle, there is also smaller tanks if need be.
you can also tune your gun down, if you dont shoot far and dont need the smack to kill something, then you can lower your pressure just fine and get many more shots on a fill of your gun, the smaller / lighter the pellet you have to push the less pressure you need to do that.