Russel gas should be able to sort you out.
I think they have a place in Sighthill in Edinburgh as well as one in Larbert, near falkirk.
I use the one in Larbert, where I live. I pay a one off cost to get the full C02 cylinder and then a monthly cost. I can't remember what the exact costs are but it's probably about £12-£15 for the gas and about £2.50 a month rental.
I explained what I wanted the gas for the first time I enquired and they had no issues supplying me, they set me up with an account and job done.
They send me a bill every month which is a bit of a nusiance. I tend to bung them £20-30 every so often to top up my account so that I'm in credit. I don't want to **** them off!
Stuart