Thanks, there isnt like "the" source, there are a lot of different sources I use occasionally for different things.
Underground lines -the ones I see & map- are usually visible by combinations of scars from construction activity, small switching boxes visible besides roads, line transition poles, older aerial images where the line was still overground etc.
The voltages are usually from company documents/system operator/system planning documents on the web, FERC elibrary, EIA generator interconnection data, public utility commission documents, etc