We've bought all of ours on ebay.

Renovator's Supply also has plenty of styles, colors, and sizes.

I buy mine on ebay or the Pasadena flea market. They are way too expensive at most architectural salvage shops I've been to. I've paid between $8 and $20, including hardware (in various levels of condition).

Try either Gainsborough Hardware or Emtek Hardware. I have not sold or seen Gainsborough in years so I am not certain of the current quality (they had been sold) but I do know Emtek is a solid product.

Thank you all for the good ideas. This forum is such a wonderful source of knowledge!

I love Van Dykes, but last time I looked their glass doorknobs were pretty pricey. I've gotten some nice glass dummy knobs on eBay in colors (and prices!) I couldn't find anywhere else. For others, I've ordered from House of Antique Hardware's website and My Knobs.com. Do a search for Fusion Hardware's glass doorknobs on the latter website and you'll find some nice designs in several colors at good prices.