Check Home Depot or Lowe's.
Start at a fireplace store. They may not have it on hand, but you may be able to narrow your search down to manufacturer and color you like from catalogs or literature. Then you can search on-line for it. Get advice about painting your item from the salesperson in the fireplace store. Depending on what you are painting you may not need a high temp paint.
Might try a farm and garden store like Theisen if in your area. If it will work for a tractor its gonna hold up pretty good I'd think.