I've had varifocals for about 5 years now. You could go to the mass market High street shops like Specsavers but the best results I've had came from Boots. In my opinion you should choose a frame which is mid to deep lens rather than the thinner variety. Protect your eyes from Infrared with the automatic reaction to darken when bright light.
Strongly doule up on the deep lens idea - maximise the size of the sweet spot.