A search on the internet will turn up lots of actual stained glass patterns (look for copyright info on these). Also Dover has a bunch of books with patterns in them (these are royalty free). These are generally black on white and are the easiest to use.
TIP:
If you are having trouble selecting the white areas with the magic wand, you do not have pure white. To fix this, go to colors/decrease and drop down to 2 colors. (leave the defaults and hit ok) This will make your image into only black and white. But in order to work with it you will need to be in 16M colors, so after decreasing the colors go back to the colors drop down menu and increase the color depth to 16M.
You can also use any image that has black coloring book type outlining very easily. Select the black outlines, then invert the selections and flood fill with white. If you are having trouble selecting the black, try decreasing the color depth to 16 colors or increase the tolerance on the magic wand.
Using any simple image can also be done by adding a layer and tracing around the shapes with black and using the paint brush tool set to 4 or 5 pixels wide. This is easiest if you have a pen tablet. If you have a mouse and find this too difficult, you could also select one section at a time and flood fill with black. Then go to selections/modify/contract and make the section smaller by 4 or 5 pixels. Flood fill the new selection with white. Do this for every section of the image and you will end up with a black and white outline graphic.
Remember GIF images are only 256 colors in depth, so if your black and white image is in GIF format, you will need to go to colors/increase and increase it to 16M before you can work with it.