Repairing antique clocks and watches is a popular hobby in the United States, but it's hard to get into if you don't know where to search for replacement gears. If you need to replace the gears in a broken clock, you will have to find new gears that are an exact match for your old ones. Before you start looking, figure out precisely what size and type of clock gears you need. This information will make your search quicker and prevent you from wasting money on the wrong thing.
A horological shop is a store that sells clocks, watches and stuff related to timepieces in general. Unless you live in a big city like New York, you probably are not going to find a brick and mortar horological shop nearby. There is certainly no shortage of them online, though. The best thing about ordering your gears from a horological shop is that the seller will probably by happy to communicate with you via e-mail to guarantee that you get the right gears for your project.
If you don't want to pay top dollar, a horological shop may not be your best option. Instead, if you know exactly what you need, try buying your gears off of eBay. You will probably find a better deal on what you are shopping for. Sellers may even be willing to communicate with you if you are unsure about a purchase. Buyer beware, though, you might not be able to return gears you are unhappy with.
You are not going to find loose clock gears in thrift stores. You will find tons of discarded clocks, though; some which work and some which don't. If you are experienced at clock repair, you may be able to find an old clock with gears that meet your needs. You can take this clock apart and use the gears in the clock you meant to repair.