Any RV parts store should be able to fix you up. I have never had a Thetford apart, but I do know that for the Sealand there are kits for repairs. It's all mechanical. I did replace a Sealand once and opted for the "higher kingdom throne" of 17 inches. Keeps my knees straight since they will not bend.
Here's one link I found on a search just now:
Search on your specific model.