Probably a Victoria's Secret or some undies catalog. Then I bought two Playboys from a buddy for $30 (one from 1979 and the 1990 or 92 Pamela Anderson issue, before her fake boobs) and was living the highlife.
Edit -- I was 12, and didn't know what the hell I was doing to myself. This
Abstruse Goose comic represents my first "Jelquing" experience pretty well.