In my opinion, eheim is one of those web efforts that can be a little on the complicated side of things but ultimately I don't care, as I'd rather they continue to concentrate on having great engineers, lol.
It's one of those that only responds to "www.eheim.com", not just "eheim.com" and then you have to get shifted to an internationally subsetted site, like north america. Once in your country/language you usually want to look in the "downloads" section to find the PDF files of the manuals. There are very nice full length manuals and parts pictures and information.
Until recently, at least in North America you could interact with a distributor via email and get parts directly but I don't see the separate little parts sub-website for that anymore and I see statements about using your dealer. I hope this doesn't make it harder to get detailed parts!
~~waterdrop~~