An opticians.
I bought glasses here which have served me well for almost a year (and still counting). It took them half a day to make them up for me and they cost around £15 including the lenses (to my prescription).
You need to have your prescription with you of course!
Its a complete mish-mash of frames, some of which are in bad shape. However, you can find some really different styles here and quite 'retro' looking things which haven't been touched in years!
I also bought glasses at a smarter looking place around the corner from here - these have been brilliant too but they are more generic and the kind of thing you could buy in the UK. The glasses I bought from Tiem Kinh Thuoc are unique as well as functional.

65 Dinh Tien Hoang, Hanoi
+84 (0)4.38241587
It's right on the north-east corner of Ho Hoan Kiem lake.
Google map: bit.ly/gTncIL

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