The most complete listing and calendar I have seen (and the one I go by)
is published in "Pow Wow Country" by Chris Roberts, a Missoula Men's
Traditional Dancer (published by American & World Geographic Publishing,
ISBN 1-56037-025-4) This book also has great Pow Wow photos and background
info on Pow Wows.
Personally, I must say I have (almost) never been to a bad Pow Wow. The
most memorable for me have been Northern Cheyenne 4th of July Pow Wow (very
traditional), Arlee 4th of July Pow Wow, Crow Fair (definitely the most Tipis,
and great Rodeo), Denver March Pow Wow (big!), Oglala Pow Wow in Pine Ridge,
Browning North American Indian Days (spectacular surroundings, Glacier Park
etc.). Others I can recommend are: Rocky Bow Agency Pow Wow (known for the
tough dance competitions, good dancers), United Peoples
Pow Wow in Missoula, Elmo Pow Wow, Billings EMC, Wind River Ethety (Arapaho)
and I'm sure many others I haven't been at.