Where do you find the block reward information for different coins? Is there maybe API, too? Or in the calculator websites people just manually enter block reward information?
Block reward is fixed. But
depends on its coins itself for every specific block this "block reward" will decrease (or we called it as Halving). Again, Bitcoin as the example. Bitcoin will halve its block reward every 210,000 blocks.
Every coin has technical info, In announcement thread (at this forum), Whattomine, Coinwarz (or wherever / something similar) you could find this "block reward" info.