Tsc store is where I get mine.
They will sell you the tank then it's just an exchange process when it's empty.
Tsc also does sales on them often.
As for the gas, argon/ Co mix for steel with a mig.
I usually get a year or so on my tank. Hopefully 2 on This one since as I haven't done as much welding
You get a welder?
Your still welcome to borrow mine