The FAA has restarted the ADS-B Rebate Program! The program will end promptly on 10/11/2019, so act soon!

https://www.timeanddate.com/countdown/

https://www.ainonline.com/aviation-news/compliance-countdown

By January 1, 2020, you must be equipped with ADS-B Out to fly in most controlled airspace.

Federal Regulations 14 CFR 91.225 and 14 CFR 91.227 contain the details