It's really easy to print decals yourself. All you need is a clear picture of the image you want on your decal, some photo editing software, an inkjet printer, and decal paper.
I usually go to Google and do an image search for the logo or marking.
The paper is available from various sources, but I got mine from the local Hobby Lobby. They're 5.5 x 8.5" sheets from Testors, either in white or clear for plastic models. Some other brands might be larger but I can usually get the markings I need on a small sheet without much waste.
I use Photoshop to work with the graphic, adjusting the size as needed. Then I create a blank document in Photoshop with the 5.5 x 8.5" canvas and I can lay out as many decals as I can fit on the sheet.
Under the print preferences, set the paper setting to photo paper, and print.
The finished sheets probably need to be treated to be fuel proof. A search can find information on what other people use, but I just airbrush on a dry coat of clear butyrate dope, followed a bit later with a couple wet coats.
Once they're on the plane I followed up with the clear coats over everything to seal them in.
I'm in the middle of finishing my second plane using homemade decals, but this one doesn't have a dope based finish. I'm not sure how well the decals will hold up without a final clear coat to seal everything down.