I know this is a very old thread, but given how rare and essential to Oaxacan mole chihuacles negro are, thought it might be worth mentioning to anyone searching these days that they're now available at the Ferry Building Farmer's Market in San Francisco on weekends, from the Tierra stand. I stumbled onto them Saturday and couldn't believe my luck. Priced the same as the dried Anchos and Anaheims, too!
Online ordering is available, though I can't vouch for it personally, at http://www.tierravegetables.com/chile.... I can vouch for the chiles, I've used their dried chiles and mole mixes in the past and they are top-notch.
Melissa Guerra in San Antonio sometimes has them. If they aren't stocking them now they may be able to give you a source if you call them.
Also, try some of your local produce vendors. Most of them that sell to the trade have a wide array of dried chile peppers. The variety improves the closer to the border you actually get.
If you live in an area with a large Mexican population you might be able to find them in a local Mexican market. Good luck, they are not easy to find.
As a last resort, you could grow them. The Chile Woman sells them as starts.