Low Cost/Free Kids glasses?

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Has anyone used a free or low cost kids glasses service? My son has - for the third time this year - lost a pair of glasses. I normally get his closer to when school starts but the pair he is wearing now is pretty scratched and beaten up (they are 2 years old). Since he has been on a bad streak, I want to get a cheaper pair. Last year I got away with $45 (lenses & a new frame) from wal-mart during their school sale, but that doesn't start until August. I have run across a few sites that offer cheap glasses: zennioptical.com optical4less.com Has anyone heard of or used either site? I have noticed that both have pretty good reviews online and through the BBB.