You could always make your own stickers, if you are handy with a computer. You could print them out on adhesive stock, or have Cafe Press do a custom bumper sticker, and cut them out from that.
If you google "custom removable stickers" you can find sites like this one:
http://www.stickergenius.com/shop/custom-prints/sticker-sheet/
I haven't tried these guys, but there seems to be several companies that can make what you want.
–Lori