You can buy surgical tubing from a medical supply store and make your own bands. I made enough for a team of 12 players to have one band each. Total cost was $50.
I bought a box of 50ft of tubing for about $25. It came in red, green or blue with each color being a different strength offering different resistance. Then I went to Home Depot and bought some foam pipe insulation to use for handles and some small carabiner clips so the bands could be clipped onto a chain link fence/backstop.
Cut the tubing into desired length, tie a small loop at one end to connect to a carabiner clip and a larger loop at the other end for a handle. I actually put some small zip ties around the loose "tail" where I made a know at each end just to ensure the knots themselves didn't come loose.