Garrison did not make the transition from minor to junior hockey until age 19. He did not play in the top tier of junior hockey in the Western Hockey League (WHL) and instead joined the Nanaimo Clippers of the British Columbia Hockey League (BCHL), where he played at the Junior A level for two years. Appearing in 52 regular season games in 2003–04, he recorded 7 goals and 27 assists as a rookie. A forward in his minor hockey years, Garrison switched to defence in his first junior year. Garrison went on to add 3 goals and 13 points over 24 playoff games as the Clippers won the Fred Page Cup as BCHL champions and the Doyle Cup as Western Canada regional champions. Competing for the national title at the 2004 Royal Bank Cup, the Clippers finished last out of five teams. Garrison had three assists in four tournament contests. The following season, he improved to 22 goals and 62 points over 57 games, ranking second in team scoring. The Clippers finished first in BCHL regular season play, but were eliminated in the Fred Page Cup Semi-finals.
