Many years ago I bought new fishnet by mail from a fishnet manufacturer. That seemed fairly expensive at the time; $6 a pound, I believe. I wouldn't be surprised if it is double that now, or more.
Then I found the free fishnet mother load of all time right under my nose, the StarKist tuna factory. They were very helpful to me in saving used fishnet that the boats wanted to get rid of. Discarded netting is a trash disposal problem for the factory, so we helped each other out.
After getting it home, the fishnet was opened out, cleaned off, rolled up and stored outdoors. It smelled "fishy". Given a month or two of exposure to rain and air it was completely user-friendly. Fortunately, I live in the country, where I can do this without offending the noses of neighbors.
Good luck finding a source of your own. Fishing ports and fish farms are good places to start looking for used fishnet. Ready-made trash sacks that can be plastered with cement should be available for this idea to really take off. Sewing your own sacks allows you to make different sizes of trash rocks, but ready-made sacks would save some time and effort.
This is a link to manufacturers of fishnet. Most are in the Orient.
A search for "netting" on Thomasnet can come up with U.S. manufacturers.