In printing bumper stickers, those who do the printing can also do the designing part. There are also those who'd offer free designs if you would reach their required minimum quantity. Since pressing the graphics onto the blank vinyl adhesive sheet is quite simple, offers for free designing is not that much to give away.
http://www.visigraph.com/custom-decals-stickers-magnets-and-labels/vinyl-bumper-stickers/
