If you want to do this yourself and make just one...
Step 1: Design the 3" x 3" bumper sticker in a high resolution format.
Step 2: Go to
Cafepress to customize your own 3" x 5" sticker.
Step 3: Upload your high resolution image file.
Step 4: Order it and receive it.
Step 5: Cut off the excess 2" of sticker.
Bam. Done. Easy.
Alternative: Copy the design twice more such that you can fit three stickers in a 3" x 10" rectangle.
You can also buy bulk orders of stickers at Cafepress, too. Or you can go through your own local print shop, too. But if you just want one sticker...