You can buy CO2 in Sainsburys,the 290 gm Soda Stream cylinders cost£4 99 exchange,I get about 6 41gm refills for my FWB C25,and 150 shots at least per charge,
The best way I have found so far is to look in Yellow Pages for a pub gas supplier,the 14 lb. cylinder I bought from them cost £8 80,and should see me to the end of my shooting life(I am old).
I strongly believe the move away from CO2 has been aided by misinformation and rumor,and not least by the makers of airguns, who could then sell us nice new PCPs.
By the way,anyone selling a CO2 5 shot pistol.
Peter