Materials for collage
There are really two main parts to a collage: the individual elements you'll be gluing down (let's call them "cutouts"), and the background you'll be gluing the cutouts to.
You can use many things as a background. I usually use printed "art" scenes I buy from the thrift store (think Robert Wood images), but I also use paint-by-numbers, or sometimes even real paintings. You can also use posters, maps, calendar images, or even a blank background. If you can't find the perfect background, you can also draw your own.
You don't need a frame, but it adds a finished touch to the collage, and a frame allows you to use glass-- most pieces look best under glass. That's one reason I like using printed "art" scenes from the thrift store. You can buy used frames with the background like I do, or buy the frames separately. If you do have a frame, remember most frames will cover just a bit of the edge of the background.
Cutouts are the individual images that are glued onto the background. Where you find these elements depends on what themes and images you like. I find well-dressed men and women in old magazines, and combine them with furniture and clothing from old catalogs, and birds and animals from nature guides. You can use newer magazines, newspapers, books, postage stamps, brochures, posters, or even online sources you print out. Look for sources that will provide images that fit the theme of your collage.
I use a lot of magazines from the 1940s, 1950s, and 1960s. By now, I've built up a library I can go to again and again. Just a few of these magazines are shown below:
You're allowed to crop
If you have a background that's a little too big, or includes elements on one side or the other that you don't like, just trim it down. Most backgrounds printed on pasteboard or painted on canvas boards can be trimmed using a sharp utility knife and a metal ruler.
Frames can be expensive to buy new. If you have a background without a frame, measure it. Then visit your local thrift store and look for art that's the same size. You don't have to like the art, just the frame.
Build a Library
If you find yourself using the same subject matter in different collages, look for sources that give you the most images you can use, and build up a library. One magazine, catalog, or book can provide images for dozens of collages.
Sometimes you'll find great-looking images that aren't complete. You can use these partial cutouts, too. Either place another cutout on top, obscuring the missing area, or use it at the edge of the collage.