Do you want a pretty large lens? Then I suppose you'd want high index. If you get a smallish lens, you can get away with using CR-39.
I have a prescription worse than -6.00, and with a 40-45 mm lens, I have no trouble with CR-39 or the standard lens material used by Zenni or Goggles4U.
I once got a pair relensed by Wohl Optics (near Philadelphia), and I was pretty happy with their work on a frame that was somewhat difficult to get the shape right. I checked their prices for higher index material, and it was pretty moderate - $40 for single vision 1.6 and $65 for 1.66. You might want to ask them first what material they use -
http://www.wohloptics.com