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
Social Media:
Facebook - event hosting platform
Instagram - promoters post event flyers here
Twitter - promoters post event flyers here
Ticketing Platforms:
AXS
Dice
etix
Eventbrite
LiveNation
Posh
Partiful
Eventim (Seetickets)
Shotgun
Showclix
Showpass
The Ticketing Co
Ticket Fairy
Ticketleap
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.