I mainly keep an eye on EyeSpyLA:
Not all of the listings are gems, but you'll pick up on this after a while.
The LA Times used to be a good source, but it's now a train wreck IMO.
The LA Newspaper Group has event pages, note different city selection at bottom of page:
BTW I wasn't impressed by the search-by-location feature of discoverourfestival.com - much too literal. Searched by Los Angeles, got Just the listings in LA proper - not any nearby cities.
The L.A. Commons site has events by area:
Festivals.com is also a 'somewhat thin' source:
Not quite a 'festival' listing, but this food truck site is improving :-):
@LA is an ancient site, haven't looked at it in detail recently: