I remember going to Ace hardware back in the day, and they said you can use some type of epoxy to retain the use of the same size drain plug.
The epoxy will fill the gap which has been stripped, and shape itself to the pitch of the drain plug. Let it cure, and it should be as good as new.
You can also helicoil it like so:
http://www.atvconnection.com/editor/.../techtip21.htm