Are you planning to get the tickets online? I recommend you to get them online, as you can get discount offers rather than buying them at booth agencies.
Check out gotobus.com for great deals!
For instance, I have looked through the website for you. Gotobus also has a tour bus to Grand Canyon from LA. In fact, they offer a 10% off for certain reservations based on the terms applied.
Details : http://search.taketours.com/tourservices/tours.do?depart=Los%20Angeles,%20CA&keywords=Grand%20Canyon&promotion_type=promotion_1;trivia_4
Hope this helps :) almost 8 years ago