Lee Valley- which mostly sells woodworking equipment- has some
letter and number stamps that are fairly inexpensive, as I recall.
They're very plain. I think they only have 1 size, but other
woodworking outfits have other sizes, both smaller and larger.
You can also look at the stamps used in leatherworking for more
options, although those have proven to be too big for the things I
had hoped to do with them...