You can make them yourself. Micro-Mark sells a kit that has clear and white paper and the sealer. As for size, You can measure the area and pick the correct font that was used then with most photo editors you can type it in a size it to what you measured. Print out 3 or 4 on a sheet so you have extra. When sealing use a couple of very light coats. Depending on the printer ink, if you spray too thick the ink could run. Make sure you use the correct paper, ones for ink jets are different than laser printers. Make sure you make it 600 dpi or higher resolution and that the printer can print that high of a resolution.
When you print them out, just print in a corner of the sheet. You can them just cut the area and seal it leaving the rest of the sheet for future decals. If you can locate a picture of the actual plane, they do make decals of letters and numbers for various scales so you can match the original plane numbers. If the plane has nose art, it can be drawn and scanned or if you can find a picture, you can make a decal out of it.
Hope this helps.