Might have been in the rebuild kit. Otherwise, from my "bag of O-rings". Sometimes you get extra O-rings with new parts, which are for other apps, so they go in the bag, especially things like auto transmission kits. I have several O-ring kits from Harbor Freight - Nitrile (black) both SAE and Metric, HBNR (green), and Viton (brown). Most colorful O-rings are HBNR, usually green but I've also seen red and blue. Often used in AC. Better heat resistance and life in oil than nitrile (bunaN).