Enter "music festivals" in a search engine. This is a good starting point for later, more refined searches because it yields general finder sites as well as larger music festivals that have their own Web sites.
Try a general finder site first when you do not care how far you have to go to attend a music festival. Good finder sites have hundreds of listings, but the festivals may be thousands of miles away.
Go to a regional festival site when you do not want to spend days on the road. A regional site covers festivals in a few nearby states or provinces.
Check professional festival production company sites. Music festivals take a certain amount of money to produce, and an industry has sprung up around the larger ones. Such sites list music events, but typically just the larger ones, not the smaller.