I have used 4clay.com in the past and it was a good stamp. However it was about $100 for a 2X2 stamp for mugs. I used it made the mugs and all is well. I was looking for an alternative and asked another potter who they use. I had a new custom mug order and I used the new place I was told about. They made jus the rubber stamp head surface without any backing or handle. The cost was less that $30 shipped and it is a nice stamp. The company is a local business Masters Mark. http://mastermark.com
I havr not bisque nor glaze fire the new mugs yet so the proof will be if the stamp was deep enough. It was not as deep as the first one I had made. We shall see. Figured it was worth the price.
All others, like jetstamps, claystamps seem expensive to me.