Not sure how many you need, but this could be done with either a laser or inkjet printer. Use white decal paper made for the printer you will be using.
Using a spreadsheet program like Excel, change the background color to black, and the character color to white. You'll have to find the proper font and font size, but then you can print, copy and past as many as you wish. I make loco number boards using this method. For reference, on an Atlas loco I use font size 4, which has to be manually entered. The local Staples near me will print a full sheet using their laser printer for me and it's like 29 cents. I've heard some stores will, and some won't. Before applying, I brush the decal with Microsale's liquid decal film, let dry and then apply. Other than that, Microscale's set 60-1046 for CR box cars has both PRR and NYC on it. You would need to paint the black background, and there are enough to do 2 cars per full sheet.
