Professor dismissed error I found in her work. I need more feedback, but the field is small
In this case, you should consider iterating with you professor again. Perhaps in person, rather through email (the explanation could be lengthy and that might be the reason why your professor was dismissive). You should also consider changing your approach, try to assume that the original research is correct and from that standpoint approach your professor for an explanation (again a situation when a personal approach might be better than an electronic one - the tone of your email might be misunderstood). You can then guide the discussion toward the issues you see.

If all the above fails, I would still hesitate to go to other people (whether in the field or not) with the attitude "My professor made a mistake and refuses to discuss it with me, I need you to confirm for me that I'm right". You course in this case is to learn as much as possible about the area and determine for yourself whether or not there was a mistake. During these actions you can and should ask and discuss your work with as many people as possible.

With original research it is often the case that even experienced researchers need to follow the above path (although their's is shorter than a student's) to state whether or not something is wrong.