Scuba shops can provide fills, but generaly you need to supply the adaptor, and they may be reluctant to fill for a paintballer.
Sites can be reluctant on filling for off site use, however contact Ambush Botley and they ought to be happy for you to come down, fill and test out on the range.
That may be a good shout about ambush, I'm tempted to get a dive tank and fill adapter but is gonna be another £100 I don't really need to spend, was hoping someone near me already had Will give ambush a shout