Stained glass patterns can be found both free and for sale online. The web is a great place to find stained glass patterns and I found one of the best ways to find specific patterns is to do a google image search.
Here's an example image search for cross stained glass pattern. What is nice about doing image searches is you get to see the actual stained glass pattern and can browse the results.
Once you have found a pattern to your liking click on the image to visit the hosting site. Be sure to check with the web site owner for use restrictions prior to printing or downloading their pattern.
Searching for more free stained glass patters using the standard search of web pages works well too but again browsing images works best :)
Here's a great free stained glass pattern resource... Free Stained Glass Patterns