I once worked for a company that used so much that they had liquid CO2 and argon and mixed their own for mig and had pure for tig pipelines all around the plant and you just plugged your machine into the pipe where needed. One thing that was done though was use argon plus 5%CO2 for mig welding aluminium as that worked out slightly cheaper than pure argon, making petrol tankers and bulk tippers.I had the same issue....I even considered using pure argon with a mixer to add Co2 for mig welding but in the end I just bought 2 bottles!