I ordered some steering bearings (Pyramid-parts) however there is no seal for the bottom bearing. i wonder if after i install it if the grease will just run out through the bearing over time.

can one buy a seal to place under the bottom bearing that is pressed onto the bottom of the stem.?

Yes seal is absolute must.
Unseating the beairng will cost you the seal. Buy new one (prolly suzuki ) and install. one more reason I went to machining the katana stems and not yanking that lower bearing.
Cool.
Buddha.