we recently had to buy glasses in phnon penh. my 17 year old son lost his glasses overboard on a mekong river trip from vietnam.
we looked at tripadvisor and found one report on a shop called Grand Optics.
this is a large shop with full optical services; eye testing with the latest equipment (no charge), and ~ 6 to 8 staff. the glasses cost US$60 (transition sunglasses); they would have probably cost $450 back in australia. i also got another pair of glasses for myself (i was planning to do this in hanoi); rimless titaniums for US$65.
cnr Norodom Blvd and 154 street
highly recommended; plus excellent & friendly staff