The Great River Shakespeare Festival, itself an amazing event that takes place from July 25-August 3, 2014, in Winona, Minnesota, also holds a sonnet contest each year in memory of Maria W. Faust, who was an avid supporter of the Festival, of poetry and the arts in general. She died in December, 2011—— her husband then had the sonnet contest renamed in her honor, and since then the contest has taken off.
An anthology of the winning poets is also available at a very low cost. Judging by the quality of the winners, it should be an excellent anthology. This link will get you to both the contest details and information on the anthology.