Poke around some fish stores, especially older mom&pop types. Sometimes they will have a rack of thinwall acrylic tubing off in a corner. I think usually in ~3ft lengths, and maybe a couple dollars each.
The TAP plastic stuff is a bit more expensive, but it's pretty high quality, thick wall, polished, etc. If you want something pretty, that's probably the way to go. It might also work better for bending, as the thin wall tends to collapse, even when filled with salt.
Depending on the diameter, sometimes hobby stores (like trains, models, dioramas, etc.) will have little packages of plastic sheets, rods, tubes, etc. Probably styrene, but I'm not sure.
If you want bigger, cheap tubes, big-box hardware stores sell those clear plastic tubes for protecting fluorescent bulbs in the light department. Usually in 4' or longer, and just a bit bigger in diameter then T8 and T12 bulbs. pretty cheap too, but fairly flimsy.