A google image search for "stained glass church windows" revealed 6150 images of stained glass church windows!
Open a new folder in your documents file on your PC and copy and paste the images that you like into your folder. You now have something to work on with your image processing software.
Many of the thumbnail images appear OO in scale and could be used without any image processing. You could simply print out the image for use in your model.
For added realism you could print the windows on transparencies.
I gave it a go to see what the result would be like and printed something out that would be very acceptable although this was with a laser printer. Most colour printers are capable of a resolution that matches the low resolution images on the internet.
In addition to Gary's comments, if you have an inkjet, you need a transparency suitable for use with an ink-jet printer - they have a special coating on one side to allow the ink to stay on the plastic.
Many churches have their windows protected by wire mesh or clear plastic to prevent damage by vandels. These often obscure the window detail completely from the outside and could save you time.
Another way to produce stained glass is to photograph actual windows with colour slide film and put several frames together to form a single window.
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