Very nice work. I took your make and resized them for a 3" x 10" bumper sticker. I put them onto a .pdf that is of the same proportions. You can take the .pdf and drop it into a custom bumper sticker template for the bumper sticker printing companies you find online. Also note that once you have up-loaded the images to your printer service you can ask them to change colours or back grounds on the images. They have the software to easily do this and usually for no charge.

The one I have been using over the years is https://www.buildasign.com/bumper-stickers