Querying the Username in a Share Data source (SSRS)
After doing some digging, I doubt you'll be able to find an unencrypted list anywhere. It's storing it in the report server, so the SSRS engine is encrypting it on it's own; i.e it's generating a key and salt in the engine code and using it to encrypt\decrypt those columns. You won't be able to query it via T-SQL, but you might have better luck using PowerShell to parse through the HTML on the reporting front end.

I believe the tag in question is the value for input named "ui_txtStoredName". I attached a picture as an example.