The best place to buy cheap glasses is Zenni.com. The ruling is unequivocal, as I have had outstanding experience purchasing glasses from Zenni. A pair of brand new, quality prescription glasses costs as little as $7! ($about $12 with shipping). Just yesterday, I got a new pair of sleek, Clark Kent style glasses to complete my in person disguise.
If you need glasses, look nowhere else. Zenni has a pair for you.