Grammarbot.io sells a grammar checking API with a generous free variant. Here I load an emacs interface is found here zevlg/grammarbot.el at github.
(use-package grammarbot
:quelpa (grammarbot :url "https://raw.githubusercontent.com/zevlg/grammarbot.el/master/grammarbot.el" :fetcher url)
:commands grammarbot
:config (progn
(customize-set-variable 'grammarbot-api-key "KS9C5N3Y")))