go to americanclassifieds.com pick denver, they have a section for steel buildings, several suppliers. I talked with a guy who used to sell them, and he recommended a company out of arkansas, the name i cannot remember however.

There's a used one for sale for less than 8k thats bigger than your requirements.

Infloor heating is the way to go, far more efficient than any other heating for the type of structure.

Keep in mind when you put it in the pour, if you ever plan on mounting something into the concrete, you need to keep it away from there.