If you guys are interested in vinyl stickers instead of normal bumper stickers, my wife has a robo-crafter that can cut vinyl. I've not verified the durability/UV proof of the vinyl, but I can get 10 feet of 9inch wide vinyl for less than $10 shipped. I think you might have to get transfer paper also, but this may be included.
If there is even remotely any interest, I'll order a roll of vinyl to experiment with. I STINK at designing, but I can use any .GSD or .VSD files, and I think it's capable of a few other formats also. I also have utilities to trace .jpgs and .bmps if that's easier.
by my math, 9"x3" stickers, having to order a roll of vinyl and a roll of transfer paper would net ~40 stickers for $20. Somewhat expensive, but for cost, I'd gladly cut them when wanted, and 50 cents for a decent vinyl sticker isn't bad.