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  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.