Cheap replacement parts for a 125cc dirt bike are available at websites such as eBay, PowersportsMax and Motorcycle Superstore or from a salvage yard. A salvage yard is likely to have the cheapest prices; however, the parts are used.
Buying parts from a salvage yard can be more difficult than shopping from a retailer, as the salvage yard often has a smaller inventory. When buying parts from a salvage yard, it's wise to thoroughly inspect the part before purchasing, as the part often doesn't come with a warranty. Buying an aftermarket or OEM part from a retailer costs more than buying from a salvage yard, but the part often comes with a warranty for peace of mind and is likely to be more reliable than a part from a salvage yard.