As Atul points out, creating a "standard" difficulty rating is not easy, as people have various level of skills, physical abilities and training. However, this problem applies to any sport that uses trails as a primary pathway. The International Mountain Biking Association has a basic primer on how to try this. Note, they only address technical difficulty, they don't assess elevation gain or trail length as part of it. That is presumed to be only physical, and unrelated to the technical skills needed.
As far as finding places, there are any number of different websites that list trails, some have their own difficulty ranking system, a lot of races, but there doesn't seem to be a single codified source. (Unsurprising, trail running for other than recreation is still a very young sport). I'd try and find a site such as Trails.com or similar that lists trails in your area. Mapmyrun will also have a lot of individually made trails/maps, some may or may not have rankings along with them.