Just check these boards always for announcement of airdrops.

https://bitcointalk.org/index.php?board=159.0

https://bitcointalk.org/index.php?board=240.0

You need to be attentive because if an airdrop has limited participants and you see it hours later then most probably, the airdrop is already close because it is already full.

As to how much you can make from airdrops, it depends on your rank and some factors. If the coins is distributed equally then maybe you can earn $5 for an airdrop and probably if you are patient enough this might become $50 or $500 if you wait for a few months before selling it. There are some airdrops that drops additional coins if you didn't immediately sell you coins for sometime.

An airdrop site that I know is this one. https://airdropalert.com/

But then the best source of airdrop would be this forum and always check the boards that I mentioned.