Where to buy welding tank?

I need a full welding tank.  

Where can I buy one?

What gas mixture is best?

How much will one cost?

I will be welding floor pans and some body work with a 110 mig welder.

You can buy or rent a tank at your local welding supply houses. Most will sell up to a "Q" size tank and only lease the larger tanks. I used to get the "T" tank from my local NAPA. Can't speak to cost as it's been a long time since I refilled my tank. Look up welding supplies in your area on Google. We have Praxair and Airgas in our area.

We typically use Argon/CO2 for Mig welding mild steel, 75%/25% mixture. There is also a mix called Stargon that has oxygen. It would be something to discuss with your supplier as to what they have and recommend for your application.