You want 30 mil magnetic roll or sheets, print the numbers out , tape them to the sheets/roll and cut using a swivel x-acto blade.
If you wipe the numbers on your pants or a rag it will remove , any particles that would damage the paint. Also remove any dust from the car using a spray detailer.
The usually come in black or white.
http://www.ebay.com/sch/Magnetic-She.../117514/i.html