The Internet. But seriously Amazon has a wide variety. I usually just collect them as I go to breweries, though.
Check out your cupboard to see if you can find anything resembling the list on http://www.beeradvocate.com/beer/101/glassware/ I wouldn't hurry for complaetion. A few visits to beer stores, events and breweries, and you will have more than you need. And the internet is a good source to find some of those that carry the logo of your preferred beer/brewery.
I got base styles from Bed bath and beyond and TJ Maxx. But for glasses with brewery/brew logo, Total wine is the best. Pricing is great and...NO SHIPPING COST
I just got these for Christmas and was so impressed I promptly bought the IPA version for my brother. He loves them! http://www.spiegelau.com/craft-beer-glasses/craft-beer-glasses/stout/
I've used true beer several times. They have excellent customer service. If you buy in bulk, web restraunt is nice. Twelve glasses for thirty isn't too bad
Get a glass each brewery you go too, makes a sweet looking collection! Usually breweries each have a pint tulip taster and seeing the tekus now.
I just make a point to grab a glass from each brewery I visit. It's a cheap, easy souvenir when traveling. And you don't have to worry about the cost of shipping or the risk of a glass breaking during transit. I've built quite the collection.
As others have said, go to the breweries for glassware. A bar near my university used to have a weekly "pint night" where you paid for the beer + $2-$4 and you got to keep the glass from that specific brewery. I was able to obtain some interesting glassware that way... and $2 for a Hofbrau 1L glass isn't too bad. So keep an eye out for any bars like that in your area. Homebrew stores usually have a decent selection as well.