Instead of searching for a high refractive index lens, you will be better off wearing 'plastic' lenses, or even thinner and comfortable polycarbonate lenses. However, the polycarbonates are very expensive.
The better option for high myopics is to wear contact lenses which have several advantages such as no diminution of image size, wide field of vision, very good correction of error as the lens is apposed right in front of the cornea etc.
However, you need to consult your ophthalmologist who will examine your eyes and prescribe contacts after testing your tear film characteristics, corneal topography etc.
And get atleast an yearly dilated fundus examination from your ophthalomologist, as high myopics will have chorioretinal degenerative changes., apart from being more prone to developing cataracts,retinal detachment and glaucoma.
Also, see your doctor immediately if you see a sudden increase in floaters, flashes of light, extreme brow or eye pain or diminution of vision.