I'm guessing you're after CO2 from the 20oz bottle size...
Make sure your tank is in test and take it to your local field. They usually charge a couple of quid for a fill, it's normally free if you are playing so go just before you leave the field for a fill and you have some gas for tinkering/plinking.
Some pro-dive shops might be able to help you (as far as HPA is concerned at least). Yellow pages in your area will list both fields and dive centres near to you.