Most of the brass gears used in steampunk fashion come from wristwatches, pocket watches, and clocks of all sizes. Thrift shops and second hand stores are a great source of cheap or broken and free alarm clocks and similar pieces. If it doesn't work and is free or cheap, you will feel fine tearing it apart for gears to make jewelry or buttons. Besides, you can look for these items online.
They are sometimes sold as watch gears to repair watches, but anymore are often sold for altered art supplies or specifically for steampunk fashion parts. There are several thousand auctions and sales of steampunk items online and you can find what you love.
Lastly, you have to own some steampunk accessories if you want to complete your steampunk look. And the Thrift shops, second hand stores and online stores are some place where you can get them.