You can find genuine Karcher spare parts at authorized Karcher dealers:
You can also find spare parts using the Parts Finder on the Karcher website at www.kaercher.com/us/home-garden.html. Simply use the search function to find the machine you're interested in, click that machine to go to its page, then scroll down to the navigation tabs:
Click on "Parts," then click the "Find Parts" button to go to that machine's illustrated parts list to find all available spare parts, and purchase them right there.