Hi piggyinpink! welcome to tPF. There are many threads in regards to your question -- try to do a search. But just to throw some websites out there, you can try Annsfabulousfinds.com. There are reliable ebayers - in fact many PFers sell their authentic bags on ebay. Whenever in doubt, post pictures of the bag in the "authenticate this" thread and PFers can help you out!
hope her info helps. basically there's no online chanel retailers. but annsfabulousfinds.com has some used bags generally...i really don't know about new but they do a score mark for you so you can be the judge if it suits you. but do do a search regarding your question.