How to find a backup of my iPhone contacts?
On your Mac, the backups are stored in ~/Library/Application Support/MobileSync/Backup/

This is covered in HT4946: iTunes: About iOS backups

If iTunes doesn't show an earlier dated backup (sometimes it saves off a backup when you upgrade from one major iOS level to another), you would need to erase and restore the Backup folder from a Mac backup (perhaps Time Machine is turned on) to get that older backup of your phone.

Also, check to see if iCloud has backups of your device. It stores several days worth of backups, so don't delay to check there in case ongoing backups of the now empty phone would erase those older backups with the data you seek.