We assessed the extent to which various sources of alcoholic beverages are used by early, middle and older teenagers, the extent to which teenagers perceive alcohol to be available from different sources and factors related to use of alternative sources. Ninth graders (n = 2,269), twelfth graders (n = 2,377) and youth aged 18-20 (n = 1,738) were surveyed in 15 upper-midwestern communities using a nested cross-sectional design. Analyses were based on mixed-model regressions, with both the individual and community treated as random effects, taking into account the intraclass correlation for each dependent variable. Analyses focused on current (last 30 day) drinkers to avoid recall bias for drinking events in the distant past. A person aged 21 or over was the most common source of alcohol for current drinkers in all three age groups; 46% of 9th graders, 60% of 12th graders and 68% of those aged 18-20 obtained alcohol from a person age 21 or older for their last drinking occasion. A person under age 21 was the second most prevalent source for 9th and 12th graders (29% of each group obtained alcohol from someone under age 21 for their last drinking occasion). A commercial outlet was the second most prevalent source for those aged 18-20 (14% obtained alcohol directly from an outlet for their last drinking occasion). Perceived availability, binge drinking and several demographic characteristics were independently related to source of alcohol used, and age interacted with several factors in predicting source of alcohol. We conclude that increased attention to sources of alcohol used by youths would facilitate efforts to reduce underage drinking.