The main people are
HPAC
If you can get to a paintball shop they can usually arrange it.
Fat Bobs list it in their online shop.
Both HPAC and Fat Bobs are around £40 to test one bottle with return post. But you still have to post it to them. (The test itself is about £20)
If you are not near any to save the extra post costs ask your local site if they can arange it, or look for local dive shops who can normally arrange it.