If anyone is in need of air tanks for PSP air guns and has access to a regulator I have about 20 Scott 4500psi bottles which my fire department has retired.
Composite SCBA bottles are only approved for 15 years of fire service use but if you hydro them they can be refilled.

Make me an offer keeping in mind that they are an odd size to ship. Check with your refill station to make sure they are ok with refilling them.

They can also be turned into an awesome propulsion system for a pneumatic spud gun if launching taters interests you!