The Christian Tradition of Prosphora is the outcome of many ancient religious practices. Here are a couple of examples of 'pintadera' or terra cotta bread stamps used in the Mediterranean region. The reproductions are for sale, but please don't use them for prosphora!
Pintadera as part of an exhibit http://web.tin.it/imedia/nuraxi/INGLESE/ukrep01.htm
Pintaderas for sale http://www.saporedisardegna.com/cp1/item.asp?id_articolo=124&ver=en
Another pintadera webpage http://www.sardacraft.com/Ancient%20pottery.htm
Other images of ancient Christian bread stamps http://www.rom.on.ca/galleries/byzantine/byzdivbread.html
And, if you can read Russian, you can visit the Russian Bread Museum (sorry, no pintaderas, but an interesting site nonetheless) http://www.museum.ru/museum/bread/