I have been trying to find the answer to this, but I have been struggling.
Finding an English copywriter is far easier than finding a copywriter for another language. Web copywriting is not just checking the language, but making it suitable for the web, which complicates things further.
I have no luck. I search a couple of sites and all I can find is proof readers. Maybe this can be enough, who knows? There are many proof-readers here, just nobody really specializing in web. I am not sure if this will be a problem.