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.