75% Argon/25% CO2 is a common gas used for MIG welding. I don't think I'd use it for CTB, though. I'm fairly certain that the CO2 content would be more than enough to trigger the hypercapnic response, but I've never tried it myself. Most places that sell 75/25 should also sell 100% argon, as there are plenty of uses for it in welding as well (such as MIG welding on aluminum, if you need story to tell the shop you're buying from).