I used sometimes the proofreading services you may get on the general platform of fiverr.com. There are many options and anybody can choose. If you developed your own business on editing, then you should marketing a lot your offer since the competition is quite strong in this domain. May be you can apply the strategy of differentiating from other, such that you may get more attention from potential consumers.
I think as an editor, you are aware that there a few professional proof reading (language Check) services and also Editorial support are available in each country and also Internationally. Depending on the author's writing and language skill one or both (language/Editorial) supports services can be availed.
There have been numerous times when I have been asked to referee a manuscript written by someone for whom English obviously is a second language. In many instances, I have sent a message with my review offering to edit the manuscript without recognition. No one has ever taken me up on it. One approach is to put it in a drawer and don’t look at it for a week. Past that, ask for help from someone in another academic department.
I do use the services of a colleague editor who provides an outsider view about the paper, and from that point we work the way that the readability of the paper is friendly and clean of errors.