My strong advice is to avoid the high street chains, who won't be able to cope with your requirement. They are bad enough on routine stuff as you can see from the post above. Look around and find a decent independent optician then go along at a time when they are not busy for a chat about frames, prescriptions and curvature. If you have a strong prescription a curved lens won't work but there are frames that are curved but have the lens in a flatter independent sub-frame, a good optician will be able to find something. Be sure however that they are confident their technician can cut the lens to fit the sub-frame, which is another challenge.
If you're lucky enough to live in Lancashire, the very excellent Junaid Ali at Ideal Eye Care near Blackburn is your man. He made me varifocal photochromic glasses in a curved Shopic frame; they were expensive but worth every penny as they are perfect in daylight or dark.
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