Hi,
There was a discussion regarding this on one of the other forums and here are some suggestions:
www.WebDecal.com
www.iprint.com
Additionally, eBay and Kinkos were also suggested.
One caution would be though to contact your insurance provider should you decide to go this route. It may very well be that using your car to advertise your business may change the structure of your premiums and coverage to that of a "commercial" vehicle and your coverage should reflect that.
Hope that helps!
