Which machine are you looking to buy for? In many cases it is good practice to replace both the chain and sprockets if it is due to wear. Chains and sprockets become worn or bedded in to each other - mixing a worn peice with a new one often accelerates wear on both as the pin to pin distance is slightly off.
Fortnine, Royal Distributing, Partzilla are all online sources. Just make sure you buy the correct match for your chain.