IN the craft section of walmart you can find it, near the yarn and embroidery boards. And it's super cheap. Fishing line in sporting goods, or also in crafts (near the beads) nylon cord which works good too.
Lol, I use it for my hammocksm, custom tank covers and the divider in the 10g ^^;
Hmm! You should find something that'll float the mesh tho, then you dont have to fit it to the tank, but it'll stay upright. Like...hmm...bah, go with balloons in sandwhich bags - or just air-filled sandwhich bags! Should be safer and low cost =D