How to localize the sources of an ERSP time series?

Hey everyone,
can anyone help me on how to localize the sources of an ERSP signal in Brainstorm?

I loaded my EEG data per subject. each subject has two conditions. each condition has ~ 20 to 30 epochs. I cleaned the data in eeglab before I loaded it into brainstorm.
I then calculated ERSP in the theta frequency for each epoch/each subject.
after that I calculated an average ERSP per condition (average over subjects & epochs).

how can I calculate the sources of this average theta ERSP per condition?

There is not a direct way to find the sources responsible of the observed EEG-based ERSP, as this is computed from (non-linear) power spectrograms. Thus a way to find the sources of the average ERSP would be to compute it directly in the source space.

One thing to consider is the size of the TF transformation for each trial (epoch), as it can get very large as source maps usually have thousands of time series (one per vertex).