There have been a few questions lately regarding parts for the Maytronics Dolphin Robots, including a recent one directing folks to an e-Bay seller who had filter cartridges for $49.
I decided I needed not only new cartridges, but two additional complete filter assemblies so I could just swap them out.
I tracked down the e-Bay seller and here is their information. The maintain full exploded diagrams (example of the M5 for the Maytronics units on their site and keep most parts in stock.
Marina Pool, Spa & Patio
They are based in Colorado and when I called they were friendly and quick to help!