Hi Smitty and welcome to the forum
Which country are you in?
You could try to contact Eheim via their website and get the o-ring direct. Alternatively find a company locally that sells o-rings and buy one from there.
The o-ring on the impellor cover holds the impellor steady. Sometimes a rubber band can be used in its place as a temporary fix. Other people make a new one out of silicon but let it dry before using it so you don't glue the cover to the motor housing.