What kind of material? Are you looking for specific patterned themes or just something that strikes your fancy?
If you aren't in a rush, the answer is thrift stores. Most every thrift store has a bedding section, with sheets, curtains, blankets, pillowcases, etc, in a rainbow of colors. Just give it a wash, cut off the useless bits, and you're good to go.
If you want more exciting patterns in smaller sizes/amounts (I'm guessing a wall hanging might be like a quilt, which uses lots of tiny pieces of fabric), clothes, of course, come in a zillion crazy colors and patterns, particularly kid's clothes and women's clothes. Yesterday I saw a top that was electric lime green with zebra-type black stripes, and it was a gigantic floaty top thing with lots of potential fabric. Kids clothes often have hearts or stripes or cartoons or seasonal themes or appliques or pictures of food or dinosaurs or animals or... the list is endless, but the amount of fabric is small.
If you're intent on specific colors or very outlandish patterns, a quilt shop will be your candy store, but it also won't necessarily be cheap. Sometimes the shops have discounts or very cheap small bits of leftover fabric though, so if you've got a specific vision, they're worth a look.
It might help other (more experienced than me!) sewers and/or quilters if you tell us more about your project. Indeed, tell everyone you know about your project and you might be gifted fabric from basements or closets (my grandmother's quilting group gives each other fabric all the time, though they usually buy it from shops).