There is somewhere in the Boro that will sell you gas (CO2) but you'll need a spare pub gas bottle to hand over. Last time I was there it was £8 a fill, but that would last me a year unless I'm knocking up something biggish.
It sounds like Wedg1e has you sorted out but shout if you get stuck.
I'm at Bishop Auckland but often visiting Darlington, maybe able to help out.
Has been mentioned - PM user Weldequipe on here if you need bits, cheaper than Machine Mart and very good service
I got a good set of gauges from him to convert my Clarke Pro-90 to take pub c02 and they have been ideal.
PM me if needed in case I don't see updates on thie thread.