Believe it or not, Crate & Barrell has a nice selection of glassware that you can purchase individually at pretty reasonable prices. That said, just about everything else in that store will cost you an arm and a leg, lol.
If you don't mind thick, heavy glasses - Dollar Tree also has a big selection, for a buck a piece obviously. They're not the most elegant looking glasses, but they hold up to being knocked around in your sink, or dropped, pretty well.
I used to get a lot of my glassware sets at Macy's after the holidays, when they mark a bunch of them down for clearance. They may still have some clearance tables/aisles up.
But as others mentioned, Homegoods/Marshalls/TJ Maxx (all owned by the same company) have a great selection of sets at low prices. I usually buy two of the same set so I have some backups when one or two break.