My circle stamp is made by Buckeye Engraving.
http://www.steelhandstamps.com/. Rachael Sudlow, a fellow Metalsmith, was kind enough to help me design this with the "s" of my first name encircling my last name. I am also pleased with this stamp--but it is only suited for flat surfaces. Buckeye had my stamp to me in a bit over a week from the time I first placed the order.
Both places do high quality hand-tooled stamps at a very reasonable price. These stamps range in price from $80-$105.