I see lease cylinders 300+ cu/ft can be had with high psi fills of 4000 to 6000 psi. Are they available in smaller sizes like 20 or 40 cu/ft?

The standard K size bottle is what you see for rent.

You can buy a 125 and get it filled by most vendors.

125 size cylinder.

60 size cylinder

20 size cylinder

I have 125 for welding gas, you have to re-do the DOT every 5 years. There is a shop in town that certifies cylinders for $35. So it is not a big deal, but some places it can be a real pain in the ass.

The make 20's 50's 60's and 80's to my knowledge. If you buy form Amazon make sure it has an actual DOT stamp or vendors won't fill it.

Thanks will look into those but I think they are rated for only 2000 psi I need 4000+.

Well shit. 5,000 psi gas cylinders do exist but you won't find them in smaller sizes.

You want what we call in the industry as a "sample bomb". Hoke, and Autoclave Engineers, Swagelok, etc. make small spun cylinders with that pressure raring but they will be $$ amd I don't know of a gas vendor that will fill them.

Here are some to give you an idea.

https://favfittings.com/sampling-cylinders/

5000 psi cylinders will be k size and larger and will be lease only from a vendor.

Now what are doing with this? What is your working fluid? (Ar, He, N2... I assume this is gas service).

If you want a small volume device at that pressure you will have to pressurize it yourself with a pump from a k bottle.

What about nitrogen paintball tanks?

You can get 90ci/4500psi tanks easily (they may make bigger/higher pressure)...that works out to 15 cubic feet at STP.

Edit just found a 180/4500 on Amazon. That's 30 cubic feet STP. Sure there are more sizes.

How much gas flow do you need at such a high pressure?  Even small flow rates will quickly lower your tank pressure below 4ksi.  

In aviation you use a nitrogen cart to raise pressure above bottle pressure.  It uses the gas to run a compressor to raise the pressure as high as you need so that you can use up bottles rather than swap them out when the bottle pressure drops below the pressure you need.  Common pressures in aviation are 2-3ksi, so you'd waste a lot of gas if you couldn't use a bottle once it dropped below 2ksi.  

It's for paintball but instead of waiting to fill and hydro testing fees I thought a nitrogen cylinder would be ideal. Just exchange no waiting or hydro testing on my end. Trying to avoid the hydro testing but that is part of the hobby I guess thanks everyone.

I have co-worker that was into super high power air rifles. He set up a booster pump that runs on compressed air to take a standard nitrogen cylinder to much high pressures in the charge bottle for the rifle.

Gas booster

This one has pressure ratio of 4 to 1 using shop air pressure. The center piston runs on compressed air and two outer pistons would  compress your nitrogen in 2 stages. Even when a standard 2200 psi cylinder is half full a booster like this could still charge a paint ball tank close  5000 psi with no problem. I think my co-worker got one on ebay for a couple hundred bucks.

I will give you a disclaimer... this is intermediate pressure equipment and you can hurt yourself and other people quite badly with the stored energy of a gas at these pressures.

If it's for paintball, why not just get your own compressor to fill your own tanks?

That is what I'm looking at but trying to cover all possible rabbit holes before I buy. The Haskell AGT15/75Gas Booster for the $650 is a grate price if working.

Do you have to have nitrogen for a gun?  Can’t you buy air compressors relatively cheaply?