SAMHSA has released a supplemental report from the National Survey of Drug Use and Health (NSDUH) which looks at underage drinking by source and how the alcohol was obtained.
More than one in four youth ages 12 to 20 report they consumed alcohol in the past month, with 31% of these youth reporting they paid for the alcohol they last consumed and 69% reporting they got the last alcohol they consumed for free. Sources of alcohol vary by age, but among younger youth (12-14 year olds) the leading sources of alcohol is family, whereas a plurality of older youth (18-20 year olds) report their leading source of alcohol is a non-relative aged 21 or older.
Regardless of their source of alcohol, youth who paid for the last alcohol they consumed drink more, on average, than those who did not pay for the alcohol they consumed (6 drinks and 3.9 drinks, respectively.) Additional highlights from the report include paying for alcohol increases with increasing age and that females are less likely to pay for the alcohol they last consumed than their male peers (76% and 63%, respectively). The complete report can be viewed here.