We have been in the same boat before. Things we tried that worked
well:
If you have a local shop that does fabric screen printing (like on t-shirts) they
might be willing to donate screen printing the numbers on the bumpers in return for the advertising on the bot. We did that one year and it was the best our bumpers looked
. Unfortunately they went out of business so now we don't have anyone here that can do it for us.
Last year we used iron numbers (I bought out Hobby Lobby of 2's and 8's). This year we ordered iron numbers from Andy Mark
http://www.andymark.com/SearchResult...ron+on+numbers
they were about the same price as Hobby Lobby just more convienient and I didn't buy them out of stock...
Good Luck!