If you feel like experimenting with actual screentones by hand you can order them online from www.deleter.com
If you do that I suggest also buying a book called
"How to Draw Manga
Super tone techniques" by Unkaku Koyama
It has the best tutorial on screen tones I've ever seen and I've seen a lot of them. But I'm warning you right now that some of the demo illustrations in there belong in porn/hentai. If it was in an american movie it'd probably be R rated.