@pastanaga - Magazines are the cheapest way of getting decoupage papers. I find old comic books are also really good. If you go to a convention, sometimes they'll be giving away boxes of comics that didn't sell.
You can also find them at charity stores and secondhand shops, as well as old picture books and so forth.
I also find it's worth having a look at craft stores, particularly those which specialize in scrapbooking. There are some amazing papers and background images to be found and often they will be quite cheap.
Craft stores sometimes sell blank wood boxes and things you can paste your decoupage papers onto as well.