From the HOZER homebrewers in Hamilton:
1. Hamilton Fire Control, Wentworth Street North
2. Dennis Evans Fire Equipment, 58 Henry St, Brantford
3. Further down the road is Patrick's CO2, 48 Page St, St. Catharines
Also, give Nickel Brook a call, in Burlington. They were filling tanks, but some people reported they may not do it while you wait.