Hanko is the "generic" name for seals used in Japan - they are still pretty common in many places where Western people would use a signature.
ShoDo works are traditionally signed by the author using one or more seals (but note that those used by calligraphers have a more specific name: 落款 rakkan) and often adding a handwritten signature.
At the moment I own a few plastic or wooden rakkan from Japan and a couple stone ones from China - if you want your own seal I wholeheartedly endorse JUN Japanese Gifts & Souvenirs
I have also experimented with having a modern inkstamp made to my specification, as a temporary replacement for a more traditional rakkan. If you want to do the same, here is what I did.
- You may also find this PDF document interesting: it's an exhaustive description of various ways you can experiment with in order to make yourself a hanko, written by Charles Morgan, a Canadian printmaker.
- Ced Nocon is an illustrator, designer, and photographer with over 16 years of creative experience. cednocon.com is his official gallery with a whole section devoted to seals.
- A two-parts article from the Beyond Calligraphy blog/site. (Part 1) (Part 2)