I don't believe or heard of any hunting or gun shops have Co2, best place to get it is from a paintball site. Hunting and gun shops will only use compressed air as Co2 is more unreliable.
No it doesn't need to be liquid Co2, but when Co2 is compressed into 3000psi, the pressure will turn the Co2 into a liquid state.
- the following video is the best that I can find to show you why a majority of tourney sites don't use it.
After reading the first sentance on the #10 post on this thread, if you want to do it right, spend another £280+ on a second hand gun £80-90 on a second hand air bottle, £10 on a bottle cover, £30-50 on a second hand electric aggitated hopper, and around £30 on a new set of grills or something with a thermal lens to stop it from fogging up
Just a heads up on the sort of cash you'll have to be willing to spend to get your kit upgraded, but if you do buy all of the above, you'll be the happiest bunny alive