19hz.info gets event info from several places. The primary sources are found at the top of each geographic section: look for the links that say "Venues List" and "Promoters List".
Other pages that 19hz volunteers look at include:
EDMtrain - event listing
Resident Advisor - electronic music news site + ticketing platform
Restless Nites - event listing + ticketing platform for Los Angeles and Austin (among other cities)
Social Media:
Facebook - event hosting platform
Instagram - promoters post event flyers here
Twitter - promoters post event flyers here
Ticketing Platforms:
AXS
Dice.FM
etix
Eventbrite
LiveNation
Posh
Seetickets
Shotgun
Showclix
Ticket Fairy
Ticketmaster
Ticketweb
Tixr
If you don't see a city or location on the site, it's because no one has come up with a source list for that region. Additionally, the site is volunteer run, so new sections only get added when people are willing to put in time to review all the data that comes in.