This place for 12 grams:
http://www.12gramco2.co.uk/
site looks dodgier than the left over half of the large kebab you bought while drunk the night before that you find salad side down on your bedroom carpet but they're actually reliable the couple of times I've used them to buy in bulk they have arrived in a couple of days.
If it leaks I can lend you my valve tool for the price of posting to save shipping one over. (bought mine via tituspb.com - he's a dealer for palmers, can supply at the same price as their website but is more reasonable on shipping).
Try putting a bit of oil on the top of the first CO2 you put in and remember you aren't piercing the CO2 by cranking down on the knob, the first shot sends the piercing pin in so you might not have to get it really tight. Also your first shot will be weak because of this but it should get up to speed after that. Oh and keep it tipped back once cocked - those barrels are huge!