Just about any jewelry supplier can make you a stamp with anything you want on it. These are designed for stamping metal. They are made of steel, would be easy to heat and use that way. It could lose its temper from heating, but that only matters if you plan to use it to stamp metal - not an issue if you're just using it as a brand.
You'd probably want to add a handle.
You could heat it with a torch if you have one, but failing that I'm sure an alcohol lamp or even a lighter would work.
A couple possible sources:
http://www.riogrande.com
http://www.ijsinc.com
http://www.etsy.com/shop/oiseaumetalarts <= this is an individual artist who does really nice work (check his sold orders to see some stamps). I'm not sure, but I think his prices are better than the jewelry suppliers. I wish I'd known about him when I ordered my logo stamp! Because you'd be dealing with an individual, he could probably make it deeper for you so you don't have to worry about the "background" showing up in your mark.
Because it's a custom item, you won't be able to order off the web site, you'll have to call. Cost will depend on the design.