I check the Craigslist collectibles area all the time for Madison, and any city within 100 miles. I try to use many different keywords to help with the search:
coins (for some reason, this produces different results)
Most stuff on there is overpriced, but someday there will be a good find. You just have to keep looking.
I have had good luck by posting a want ad on there though, with somewhere around 30 deals done in 15 months.