Hi meanjean, any pet store should have the replacement biowheels as well as the little blue thingies that are on the arm that holds the biowheel in place.
The reason I mention the little blue thingies is because once my biowheel stopped, I tried everything to get it going again and guess what, one of those little blue thingies was missing!
Other things to look for (and have already been mentioned but these have happened to me personally and might help...)
The impeller. Take apart your whole get up, take out the impeller, and rinse it under hot water to dissolve the crud. THEN, take some tweezers and pull hair outta the thing...I have two cats and long hair myself, and you wouldn't believe what gets collected in there....this stops up my biowheel all the time. Now I pluck the hair out of it at least monthly.
Move around the filter cartridge. Sometimes mine is very sensitive to the location of the cartridge and just a move or wiggle will get the flow going again.
Have you rinsed the bracket that the biowheel sits on?
Good luck.