Not sure if this is what you are talking about, but something like blockchain.info has? but for all coins?
https://blockchain.info/apiI dont know where the calculator sites get their info, but lots of times, it seems outdated. I would guess running own node would be only way to get realtime realtime without going through service like blockchain.info
Yeah its a major pain in the ass, I'm needing it for all SHA-256 coins that are currently being mined.
I resorted to writing a wrapper to pull statistics from bitinfocharts using Beautiful Soup and then parsing the html for the data. It works but I really did not want to do that as it adds to my coding load lol.
Have you looked at coinwarz and whattomine's APIs? Easy enough to pull down JSON and play with the data.