Do an
ebay search for "CDV" and you'll pull up a boatload of them.
They're not necessarily expensive, although depending on the subject matter of the photo, the location of the photographer, and/or the cancel, they can bring relatively big bucks.
I only have a few of mine imaged thus far. I have another 40 or 50 all with handstamped or printed cancels that I need to get around to processing at some point.
If you want to view a magnificent exhibit on these, Bruce Baryla's 8-frame exhibit is phenomenal and extremely educational. I've bought some of his duplicates over the years, but he has some truly exceptional items:
http://web.newsguy.com/bruceb/london/exhibit01.htmHere is one of my favorites from my collection, that I picked up from Richard Friedberg at Chicagopex in November. Not only a margin imprint single, but a scarce stencil cancel to boot:
