Find your local HB club. Join. Make it known that you're looking for glassware. Politely turn down 90 of the 100 offers you get to take glassware off peoples' hands.
Alternately, haunt Goodwill and other thrift stores.They're not so good for specialty beer glasses, but are very good for wine glasses and brandy snifters, as long as you don't mind mismatched glasses. Occasionally, you'll luck out and find an old style pilsnener flute, although it probably will advertise a brand of beer you'd never willingly drink.
I haven't done it, but I'd also imagine that flea markets and Craig's List might be good sources to get cheap glassware.
My "go to" beer glasses are:
1) Sam Adams "nonick" glasses. I like these because they're a good compromise between the standard shaker glass and a weizen "vase", fit nicely in my hand, and have etched nucleation points which helps head development. Sometimes these are given away by SA brewery reps at events. If you see them in your local good beer bar, you might also be able to sweet talk the management into giving or selling you a couple.
2) An assortment of 2-oz taster glasses. Nice for when you want to do parallel tastings, share out beer samples among friends, or if you're wanting to practice BJCP-style beer judging. Also good for HB club meetings when you don't want to drink too much. I've gotten way too many of these by volunteering at/attending beer festivals over the years.
3) An assortment of 6-8 oz. taster glasses. Ditto.
4) 6 or 8 oz. stemmed tulip glass. Good for bigger, fuller-bodied beers where you don't want a full serving, or for splitting a bottle of beer with a friend. The bell helps retain aroma and head. A friend of mine uses a small (4-6 oz.) spirit snifter as his preferred testing glass for this reason.
5) 12-16 oz. brandy snifter, goblet or tulip glass. Good for Belgian strongs, Berlinerweisse, Saisons and other aromatic, big-headed beers. These are commonly used to serve Abbey/Trappist beers in good beer bars. Again, you might be able to sweet talk the management in to giving/selling you one or two. Some champagne glasses you see in stores might also fit the bill.
6) 16 oz. hefeweizen glasses. Good for all beers with big frothy heads, not just hefe.
7) 12 oz. lidded ceramic "maas" mug. Mine was originally sold by Avon and had aftershave in it. I bought it at a flea market (sans aftershave) years ago. Good for outdoor drinking when I don't want critters or gunk in my beer. I just wish I could find more like it.
Everything else just gathers dust, or sits in the china cabinet because it looks pretty.