I've posted about geocaching and letterboxing to
craftgrrl a few times now, and I thought I'd share
Sissy n' Cr's store with you all. They are a resource for getting custom-carved stamps. You send them the lineart, and for a really cheap price ($3 for up to 2 square inches) they'll laser-cut your art into a stamp material. They send you the unmounted stamp with adhesive, and then you just stick it to a piece of wood. Additional square inches can be purchased for $1/per inch.
I had these turned into stamps, and they cost me $3 each:

I was incredibly pleased with the results, and will be using them for any further stamps that I need.
And even if you don't letterbox or geocache, it's always nice to have a resource for a cheap stamp carver, usually things like this run $16 or more.