Airdrops happen when via a hard fork attempt. (Anyone who claims different is wrong, the bounty campaigns love to confuse the term!)

The best way to find an airdrop is the following:

(1) Cruise the news sites to identify which hard fork attempts may be scheduled (All 2018 Hard Fork Attempts)
(2) Buy the cryptocurrency that will be the target of the hard fork attmept
(3) Hold this cryptocurrency through the hard fork date
(4) Claim new ICO currency that was granted via the airdrop

Good luck!