Training is a tricky one. Most coursework for family creation is generic, whereas MEP fittings/accessories often care about modelling orientation in Family Editor for proper interaction with the Revit MEP network tools. Because of this, the people that would know the most would be manufacturers of pipe fitting/accessory content, but they aren't necessarily in the market to train.
If you haven't taken the steps below, they are a good starting point:
- Check the Revit out-of-the-box families. Geometrically they get pretty close. More importantly they'll give you an understanding on the directions connectors need to point to properly design elbow/tee/wye families to be compatible with MEP design.
- Check manufacturer valve families on BIM repositories, and compare their methodologies to #1. Try them in a project as well to see if they work as intended.
- Watch some Autodesk University presentations on pipe design. Not necessarily for the family modelling aspect (very few get into that), but to get a sense on how different industry members actually *use* the families versus how you or your company does. This helps check against personal bias, and opens you up to understanding other use cases that you can then account for in your own family creation process.