Ya got seveal possibilities here. http://www.whoopieslings.com/
OTOH, if you wanna keep the budget reasonable, all straps will stretch, but poly will stretch less than nylon - you can get poly at a sewing or outdoor store. I get mine at "campers village" here which sells tent bits too.
the other thing that will make a difference is how you tie the hammock up to a tree. if you use the right cinch it's pretty easy to get it nice and tight.
All stuff stretches some, the first couple of times you use it you may need to put it up, sit in it, then retension. after that you'll be fine.
Do you have Ed Speer's book (which shows how to do this well with a strap hammock?)