The key words to search for when looking for dealers are “Herman Miller” which is the company that manufactures the Aeron chairs. There are plenty of used office furniture dealers who specialize primarily in used Herman Miller goods, and not only will you find the chairs you are looking for there, you might find the office set of your dreams!
Though accredited furniture dealers are probably the best way to get your used Aeron chair, don’t dismiss pure chance. Get familiar with the look of an aeron chair and then try searching them out at yard sales or eBay. If you recognize an Aeron office chair but the vendor doesn’t, that could be the best sale of all!