Answer:
Most Disney Channel auditions are only available by appointment through a talent agent. The best way to find legitimate auditions is by getting representation, ideally through an agency that regularly works with Disney Channel or ABC Television. You will most likely need previous acting or singing experience before the agency will agree to represent you.
There are several websites dedicated to posting audition notices. Most of these auditions take place in the Los Angeles, CA area. Because Disney Channel runs some separate programming on its international stations, there are occasionally casting calls in Canada, England, Australia, New Zealand, etc. International auditions are also available for extra casting in films.
You might also research what independent production companies are involved with Disney Channel, such as It's a Laugh Productions. These companies sometimes hold auditions under their company name, rather than the Disney Channel name, to avoid crowds of unqualified auditionees who are simply looking for their "big break".