I generally make my own screen printing art using Adobe Photoshop. On my Screen Printing Artwork page, I show you how I made one piece of art.
Graphic art for screen printers is widely available. Lot of different things can be used. You can use clip art, photographs, whatever.

You can make your own art using any computer graphics program you have available. The paint program that comes with Windows can be used. My own choice is an old version of Adobe Photoshop. I love this software, and the use of layers allows cool images to be easily made. I have also used Microsoft Word to make text art.

There are plenty of sources of clip art that can be used. If you are planning on makng shirts to sell, make sure you have the rights to use the art for that purpose. If you are making a shirt for yourself, you probably shouldn’t have any problems unless you use some trademarked image, and happen to draw the attention and tick off the trademark owner. I have read this has happened with people using Disney images.
If you find single color images, then you are all set. If the images are multiple colors, then you will need color separation software. Color separation can be costly, involved, and is not for beginners. It would also require the use of a multi-color screen printing press in order to print it.

You can create screen printing art from photographs. If you want to print the images in multi-color, again it would require color separation software, and a multi-color printing press.
Another less expensive option is convert the photograph to a halftone image. Halftone images are like those black and white images that you see in newspapers. They are made up of little dots of different sizes. This can be done using Adobe Photoshop.
And even another way to print a photograph is to use a high contrast photograph, and convert it to two color. This is trickier to do. The photograph needs to have the right balance of dark and light shading, and then it stills requires a lot of exerimentation to get it to look good.
When printing black and white photographs, it works best if you are printing white on a black shirt, or black on a white shirt. If printing white on a black shirt, you would initial print a positive image (the black parts are printed black). If printing black on a white shirt, you would need to print the image as a negative. The white parts print as black.