If you want to go cheap, get some Rachet straps at Walmart. Look for the 1500lb ones.
If you want to go for the most awesome straps ever, order webbing straps from warbonnetoutdoors.com.
I love the Warbonnet straps, but have hung just fine with the $6 walmart straps. 300 lbs.
Avoid the Eno Slapstraps, they are worse than worthless.
I also made the mistake of trying nylon straps. I was convinced I would have to start packing a ladder to get my hang high enough not to turn my DN into a beautifully-gathered ground cloth! The AHE system did the trick for me. Quality stuff and I love the whoopie slings!
I have straps from ArrowHead & a couple of other vendors.
If you don't mind spending a little more money, you could also get a set of Python Straps from Kammok or some Atlas Straps from ENO. They don't stretch and they have several attachment points along their length.
If you can't afford the ratchet straps yet, you can keep trying your current one. Just remember that the more line on each end means more stretch.
For instance, if you have only 6" of strap from the tree to your ENO at each end, there's not much line to stretch. If you have 4' on each end (hence a 17' gap), then you'll drop a couple of feet from the stretch.
Pre-stretching the line a bit can help. Set up the hammock, then put your body weight on it for several minutes. After it drops, re-adjust your straps more tautly.
I'm a heavy hanger. I find seat belt from the local wrecker yard to go around the tree and whoopie slings to the ENO DN works perfect with no stretch. I don't use anything else and I sleep in mine every night.