Egg crate. It's located in the ceiling tile/light louver area in HD/Lowes. Roughly $12 for a 24" x 36" sheet. Cut it with tin snips or just use a hack saw and hack away. Be careful though, plastic shards will be flying everywhere when you cut it.
This is probably your cheapest option. Not mesh, but it works well. If you have some 10" SA cichlids or other large strong fish you'll have to stop it from swinging under water like a cat door.
10g - 3 Feeders, 1 Bolivian Ram, 1 Bluegill?
40 Breeder - Eastern Painted
50g Rubbermaid - Mississippi Map
I work at an LFS..