>Where can I find statistics for underemployment?
You can't, really. How can you define it rigorously enough to put a number on it? Is a pianist who earns her money by driving a cab underemployed? Does the answer change if she's a good pianist or bad? If she's a jazz pianist or classical? If she lives in Boise or Brooklyn?
In the U.S., there are numbers for what the Bureau of Labor Statistics calls "alternative measures of labor underutilization" at <http://www.bls.gov/webapps/legacy/cpsatab8.htm>. These include people who want full time work but can only find part time, and people who've dropped out of the labor force in despair of ever finding a job.
Doug