It will be difficult but not impossible to find a turtle which fits your needs.
There are some main factors you need to focus on: temperature and water quality. The temperature will be important for your turtle, the water quality for your fish.
You need to find the kind of turtle, which will survive in the temperature region you live in. If you decide to ...
Yes, there are a decent amount. In fact, I have a specific species that might work for you: Texas river cooter (Pseudemys texana); they do not eat fish but instead are herbivores. Just keep them well fed.