Garage sales and flee markets do on occasion provide fruitful hunting grounds, but by and large you will find the number of clocks and their variety very limited. The other problem is that the condition of the clocks is often poor and their running condition at best questionable. This is not a perjeorative criticism of such sources, but merle my experience in hunting for antique clocks. If you don't care if the clock runs, or if the movement is original then you may be satisfied with the occasional find. However, if you want a nice antique clock that has its original parts and is in good running order then you should probably spend your time looking elsewhere.
Auctions are fun and can be a fine place to purchase antiques.
Clocks can be found at general estate auctions as well as special sales of clock collections. At auction all items are sold as-is , and in general all sales are final. This means that you should check out the items before the auction begins and then decide on a maximum amount that you will bid. It is easy to get carried away with the biding, but that is a matter of exercising self control. General antiques shops are another source of antique clocks.