i used Zebco omniflex 30 lb. test fishing line at walmart the 250 yard roll should be 2.50 + tax
I've used long heavy boot laces.
Assuming you want low or no cost options, go down to the local big box hardware store and get some of the free plastic twine that they use to keep loads tied down with when you leave. It doesn't stretch (that's bad) or have a twist (also bad) and you can adjust the thickness by pulling some of the 'threads' off of it.
I would try 1000-Pound-Test braided deep sea fishing line, with some sort of protective covering, like heat-shrink tubing, to keep your fingers on your hand when you pull back the drawstring.
it depends on the size of the bow. if it is a small bow you can use normal string but if it is larger you could use paracord!
Kite string should be good - just be careful, because it is thin and cut your fingers.