Craigslist is the best place for me, but I find it helps to use certain keywords (and their alternates or misspellings) to aid my search. ie: cabinet, cabnet, cupboard, dovetail, dove tail, mahogany, mohogany, old, vintage, antique, handmade, hand made, heavy, classic, etc. Also, search for the brand names you like, or if you can identify the style or era of furniture you like you can use that too. I usually set certain parameters in my search to narrow the results, such as a minimum price of a dollar - a lot of spammers will not put a price and that gets rid of them. If you click on the category drop-down menu once you have pulled up your search on craigslist, you can choose 'Furniture-By Dealer' or 'Furniture-By Owner', these are not options you can see on the main craigslist screen, so a lot of people don't know you can weed out (most of) the dealers with this simple step.
You may also just do a search of vintage furniture and your location in google and see what you get. I just did that and discovered a cool new shop I didn't know about!