Never heard of "bumper sticker paper." there is adhesive-backed sticker paper, but it's generally not waterproof. Some sites online can print custom ones for you. If you wanted to do it all yourself, I would recommend printing your message on a normal piece of paper, laminating it (like $.50 - $1.00 at kinkos) Be sure not to cut through the paper, it should be sealed all the way around with a margin of just lamination.
Then you just have to figure out how to stick it to your car, If you want it to be permanent, use some superglue. Generally any kind of tape will fall off pretty quickly due to sun, water and temperature changes.