I attempted to create my own zip-a-tone using Photoshop and then printing onto clear adhesive paper. Photoshop is limited in the pattern department. To do it, I actually created a square with fill that I either scanned, created or experimented with in Photoshop. This took forever and the results were ok.
I think the scanning the pattern idea is best as long as you can make it fit the area you want filled.
I've used things like old newspaper dot patterns, things from drawings or photos I liked and my own drawing. I have heard that an artist once xeroxed a picture that had this amazing texture from an Elvis album cover and every now and then you can tell which area he's using if you are familiar with the album cover. I check into that more, but nothing is taboo if you blow it up or cut it out and use it for fill or tone.
The only thing that can be a pain in photoshop is printing it. Halftones do not show up as well unless you know how to break them down for printing purposes and seperate them out. Be careful about grays that are not black pattern or fill. Same for the scanning, do not scan it in as b & w. Use halftone or color and then break it down in Photoshop after the scan. Hopefully you have a decent scanner. Anyway, I'm done. If I can think of anything else I'll let you know and let me know what you end up doing and how it works out for you.