I was recently walking around the toy section of my local walmart when I happen to stumble on a custom decal kit, made by testors which can be used with a inkjet printer. It does come with one clear decal sheet, and one white decal sheets. Supposidly, the sheets are can be re-ordered from testors i believe. Here's the product link on the Testors website:
http://www.testors.com/catalog_item.asp?itemNbr=2255