CO2 is probably easiest if you can find a friendly pub landlord to get you the cylinders.
Disposable can only be recommended if you're only ever doing a small amount of welding.
For a proper argon/CO2 mix in normal cylinders, you're looking at one of the rent free places. I use weldinggases.co.uk personally. Also it seems many local motor factors offer similar, handy as you can pick up in person. I find the place I use good, but it does add cost in deliveries/pickups.