You can try to get the craft mesh from a craft store/walmart/online, you can melt pieces or tie with string/fishing line together to form a size appropriate to your tank or use one of the cheap poster board picture frames - they have the clear plastic protective cover, take it off and cut it to size since they should be large enough. Get book report binders and use them to hold in place on the side of your tank, if you have issues because it is so large a paper clip rearranged in size will hold the top of it snug to the top and side to keep it from moving.
DIY dividers are pretty easy to make for smaller tanks, little more work for a larger tank. DIY also holds up much better than the purchased dividers, they wear out and don't last at all. Somehow the dividers we ordered wore through, literally got holes in them and one of bettas got through and beat the crap out of my other betta.