All States Ag Parts and Reliable Aftermarket Parts have been a good resource for both new and used for me. eBay can be good, too, especially new old stock. I took a chance and bought a running takeout tractor engine sight unseen from Worthington Ag Parts and it's been solid so they get a vote of confidence from me. I'd have a hard time running my old jun-, er, iron without being able to buy parts online. If you have part numbers, just start punching them into the Google machine.