Pretty much any competent optician, Frogskins aren’t complicated to glaze, the lenses basically just push out, and they’re not far away from Wayfarers shape-wise. I had a pair done many moons ago by a local place with no problems at all.
Keep hold of the original lenses, put them somewhere safe you’ll remember, just in case you want to put them back to sunglasses at some point; I lost mine, but found someone who does absolutely superb replacents for not a lot of dosh, and with Iridium-type coatings, too, I put in +Red Iridium, which was never available in the very early 90’s when i bought mine.
These are the people I got my replacement lenses from, worth keeping a note of just in case:
http://stores.ebay.co.uk/revantoptics