If it is just lettering and numbers any sign shop has an assortment of fonts, just select one and a size and they should be able to help you out. If you want a special design it could be a different story. Depending on the cutter/plotter the shop uses, they need a specific type of file, a vector file like .DXF I use Corel, it is friendly to my cutter/plotter. Probably the easiest way is to use MS Word, print out what you want and take it to a shop. They "should" be able to match it.