Arnold,
If you have the original boxes, they locos come with a spare set. Do you still have those? When we have gotten a new loco, I take those tires and put them in a new Ziploc bag with the name of the locomotive and then put the tires in a baseball card box I use for various types of parts.
Assuming no, the best thing to do would be to use the MTH website to find a MTH service center and call that store to see if they have them, or to call MTH directly. Call after 12pm, don't email.
I hope that helps. (I have enjoyed your posts that focus on the fun of the hobby and not whining about something being broken.)