Try broadening your search to more than just bipolar disorder. A lot of PI's are into more transdiagnostic approaches, so it has been common to study psychosis spectrum related disorders like schizophrenia, schizoaffective, bipolar, and major depression together in the same grant. There's a consortium group that focuses on Cognitive Neurocomputational Task Reliability & Clinical Applications that is running an EEG study on that transdiagnostic population-- hope this helps! Other than that, what the other poster said is super valid too. Familiarize yourself with the literature and what topics interest you the most and you'll start seeing names popping up over and over or being cited a bunch. Here's a list of the PI's associated with the consortium I mentioned above.
Deanna M. Barch
Washington University in St. Louis (WUSTL)
Cameron S. Carter
University of California at Davis (UCD)
James Gold
Maryland Psychiatric Research Center
Angus W. MacDonald III
University of Minnesota (UMN)
J. Daniel Ragland
University of California at Davis (UCD)
Steven Silverstein
University of Medicine and Dentistry of New Jersey (Now at University of Rochester)