The other guys are right about the products but if you are having trouble getting them you can try this suggestion. I work as a therapist at a clinic and we have the exercise bands that we use for the patients. Maybe look up a local clinic or sports medicine office and ask for one of those bands. It will cost you a few dollars and they come in various levels of resistence. Be sure to exercise both arms as on side is pulling and the other is stabilizing the bow. Do the exercise as if you were pulling back a bow string with on end of the band tied to a bed post or door knob. Do the exercises from different angles so get all of the muscles which are involved(arms, back and chest)
Good luck and stick with it.