I was looking around the web today and I found this site that makes custom signs, magnets, bumper stickers and banners for businesses. Visit: http://www.buildasign.com/
I like the site because you can design your sign right there online much like VistaPrint allows you to! I enjoyed checking out the magnetic car signs/car bumper stickers page and am playing with a design right now to have printed through them for my own business (Custom Gifts by Talina).
I don’t see prices listed on the site so I get the impression you have to create your design before you can get size and price information. However, this is the most user friendly site I have found and it makes creating your own sign, banner, bumper sticker, etc a snap! This is way quicker and easier than calling or visiting a print company to have your custom sign built!
If you are doing craft sales, vendor displays or if you have your own store you should check this site out! Also, if you have ever wanted to place a sign in your yard or have a business decal or bumper sticker on your car this is a great resource!