In 1999, Sostorics made her Team Canada debut with the Under-22 National Team at the 1999 Christmas Cup, winning a gold medal. In 2000 and 2001, she continued to play with the Under-22 team, capturing gold medals at the Nations Cup tournament both years. She served as team captain during the 2000–01 season. Her performance at the Under-22 level earned her a spot on the Senior Women's National team for the 2001 World Women's Hockey Championship. At this tournament, she contributed two goals and an assist as Canada won the gold medal.