Cheapest place I have found is izoomgraphics.com you can get a basic set for about $30. You can get some really nice ones for about the price that most other places charge for a basic set of numbers.Originally Posted by Ataim
You can also use Magnetic stock from a hobby shop and print nubers on Avery waterproof mailing labels. The mailing labels can be found at OfficeMax. I don't think Office Depot carries these. I could have it backwards though. These will not work with an inkjet printer, so you will either need to have a laser printer, or go to Kinkos to print them out.
Something that others use temporarily is painter's masking tape. This can be found at any Wal-Mart, and I have yet to see it damage anyone's car.