Maybe co2 powered gun as FWB C20, there is one at the "Sale" section , if you have possibility to load cardridges with co2 of course.
Pubs are a good bet for co2 if you don't want to shell out the £70 or so for your own co2 tank.
I filled my C20 at my local pub, paid them £1 a go for 2 cylinders. For 4 nights shooting a month i had to fill the cylinders twice so reasonbly cheap.