There are a few good companies out there that have a decent selection of Jewish themed stamps, but you have to admit that ther isn't the selection we'd like to have. This is where you just have to be creative.
For a while now, I've been searching the Christian Rubber Stamp websites for stamps I could use with my cards. You can find bibilical passages (psalms, proverbs, etc), biblical fruits (grapes, pomegranates, etc), and other appropriate stamps. (Keep in mind if they contain the word G-D, that most Jews keep to the thought that the page cannot be simply thrown away, and has to be "disposed" of properly - thus why some synagogues have huge storage areas for old prayer books etc - they aren't thrown away).
Here are some links to get you started:
- Biblical Impressions - http://www.biblicalstamps.com/
- Faith Meadow = http://www.faithmeadow.com/
- Catholic Rubber Stamps - http://www.catholicrubberstamps.com/ - more Catholic, than Christian - but there are a few that could be used.
- Verses Rubber Stamps - http://www.versesrubberstamps.com/
I hope this helps you in your journey. Please let me know if it does or if you have additional comments.