There are several freelancers available here whom a student can directly contact, off the forum, for such services.
I've provided at least a dozen of them over the years, in very different areas, and with good results. Typically, I'm able to do this because PhD candidates usually provide much of the source material, including the institutional dissertation guidelines. People often mistakenly assume that one actually needs the same substantive
knowledge in the field as a PhD candidate to write dissertations, which just isn't true. When the client provides good source material, a dissertation becomes nothing more than a high-level writing project with a very particular structure rather than any kind of "test" of the author's own knowledge in the field. I'm currently working on one in the nursing field for which the client wasn't able to provide much material; but it's a topic that I'm able to research. That isn't always the case, and there have been others that I had no choice but to decline, precisely because I'd only have been able to write them relying on sources provided by those clients.
While there are companies out there that claim to have human writers work on the PhD papers of their clients, most of them will mislead you into thinking that their writers actually have the level of education that you are looking for
This is true, simply because few (if any) commercial essay sites maintain any writers on their staff who can write at this level. By the time we're able to do so, almost all of us have already stopped writing for essay companies because we've managed to establish our own independence and private clientele. When I was writing for essay companies (until 2013), there were only two or three of us (out of the hundreds of writers on staff) who could ever be trusted with this kind of work, and this included one subsidiary company whose name actually included the word "dissertations."