There are two resources for this that I am aware of, both provided by the TeX Users Group (TUG).
The first is their TeX Consulting and Production Services page. There, numerous organizations/individuals have listed themselves as providers of TeX-related services. There is a nominal fee due to TUG in order to be placed on the list. While none of the listings are specific to illustration work, several of the listings indicate graphic/design capabilities and/or graphic designers on staff.
The second is the
tug-consult mailing list hosted by TUG. The list archives show no messages from the past few years, so it seems to be not active lately. But previous messages included RFQ-type messages and other discussion. This could be a possibility to get in touch with somebody, even though it's not been active for several years.
And an aside about the question 'not' asked: Is it remotely possible for someone to make a living doing this?
I'm just getting started on a similar endeavor myself. I never planned for it to be my entire living, so I don't know if that's possible. But if it is (and of course this depends on what your personal definition of a living is), I think it would be difficult. I haven't found enough people needing these types of services to create a full-time workload. Of course, I haven't tried that hard, either (yet). I suppose it would be possible, though. My advice is: start small, do good quality work, and slowly build your network of clients. Don't limit yourself to working with just authors during the writing process or just publishers at the final compositing phase—mix and match to even out the workload. Working with individual authors can be slow-going and possibly change-intensive. Working with publishers, you'll get lots of work in big chunks and they usually want everything done yesterday. Every so often, evaluate your situation to see if 'quitting the day job' might make sense for you.