i hav espoken to many dive shops and they all say that it is difficult to fill 310bar (full fill or 4500psi). most tourney sites have fill stations for air and often have something like the genesis machine for 4500 fill. if you are looking for tanks then get a 3000 tank, much cheaper but get a din fitting and a 4500 din fill adapter, you will always get 2000-2500 bulk fill from the tanks and you can always quickly top up with another tank of from the air fill station.
email me if you want a 3000psi 9L tanks as i know some people who are selling them for £40 a piece.