Ello.
Yep,lots of things to know.
You really want, if you can afford it, 12 litre 300 bar (4500psi) tank and fill station. This will be about £230.
Try s/h but dont think you will have much luck, I didnt.
I use a 232 bar (3500psi) 12.2 litre bottle. Mine IS a divers bottle, it has a diving type valve, known as a wet valve on it. These bottles need to be tested inline with diving regulations.
If you get a Dry valved bottle, that cant be submersed, you get a longer test, I believe.
Try the Airgun magazine Airgunner, there is an advert for refurbished tanks with all the bits in there.
I dont know if you drive or get chauffered but read my post WARNING.
Gaz