It's not just that maker (Dometic), it's that specific model (300) that is having issues.
You can contact Dometic directly to have a new toilet either sent to you, or to a RV service center near you. When I had a toilet replaced (different issue), I arranged everything with Dometic and had them send the toilet directly to a local service center. They received the toilet and I made an appointment with them. They billed Dometic directly for the time. (Now, I had them do other stuff- so they got me there.
Here's the Dometic web site: