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, New York City, and Austin
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