So I try to utilize a combination of art I've bought directly from the store or from an online shop. Deviant Art is my favorite online shop, but there are others, such as All Posters, Art.com and many others.
But because I want to change things up frequently, I need to save money, and I've found that estate sales, garage sales, rummage sales (especially church and school rummage sales) and thrift stores enable me to buy art at bargain prices. Other people want to change their walls, too, and their castoffs might be my (or your) next wall art find. Here are some of the things I've picked up at such sales (apologies for some of the images. I took them with my camera, and I'm not skilled enough to get rid of the reflections. Where possible, I found a duplicate image online, but that wasn't always possible).
This image doesn't do the art justice. It's actually a poster from the America's Cup 25 in 1983, and there is faint writing on the image. The frame was broken, so we re-framed it.
I believe that this sunflower painting by Anne Worthington was formerly an IKEA item, but we bought it for five dollars at a garage sale.
This one came matted in a gaudy gold frame (there's also writing beneath the image on the matting), and we didn't have the right sized frame, so we cut the matting a bit and re-framed it. I've shown both the original and our framed version (complete with my awful photo with reflection and washed out color).
This is on Irish Linen, and someone framed it.
A National Park poster (we framed ours, but this is the poster sans frame).
It's always an adventure at sales, especially estate sales where you can sometimes see the art hanging on the wall. I doubt that we've ever paid more than fifteen dollars for a print (well, maybe twenty for a really large one). That has enabled us to move things around when the mood strikes.