call airgas supply they have 3 grades of N-butane in large bottles (99.0% , 99.5%, 99.99% wanna say they are $120, $136, and $762) It beats the hell out of all the cans to buy. You have to have an account with them though......but who doesn't know someone who welds??
p.s. here's a funny joke I heard the other day:
Q: what do you tell someone with a 3rd grade education???
A: Nice weld!!!