The answer to that question is probably going to change with time, but right now authors and publishers can find industry pros through one of the several startups that have launched listing services.
First up is Smashwords. While they are primarily known as an ebook distributor, for the longest time Smashwords has been maintaining a list of ebook formatters and ebook cover designers. Those are just two of the skills an author might need to hire, but if that's all you need then you need not look further.
Next up is Bibliocrunch. This 3 year old startup is more focused on offering a concierge/mentoring service, but it does also offer a marketplace where authors and publishers can meet and hire freelancers to work on a project. BiblioCrunch offers a free membership tier as well as a paid membership with more services and support.
Just launched into beta last month, Reedsy is one of the newest of the services mentioned here. It's free while the bugs are worked out, and offers a curated list of freelancers.
This is one of those startups which is hard to quantify. It doesn't offer a marketplace so much as it offers a listing service for freelancers, while at the same time building a community through local events and around its blog. With more categories than you can shake a stick at, BM can connect you with freelancers who can do almost anything.
Blurb has made a name for itself in distributing POD books, especially graphics intensive books like coffee table books, but in the past year Blurb has expanded to include ebook distribution and other services.
And last week Blurb launched a skills marketplace called Dream Team. That marketplace is still limited to a select number of freelancers, but it should grow in time.
This is a mailing list for copyeditors, and it also maintains a list of freelancers editing services.
A belated addition to this post, HelpuPub offers a directory listing of reviewers, designers, editors, and more.
And last but not least, we have White Fox. This 2 year old startup is a services provider and not a marketplace, so it doesn't quite belong on this list. But it is similar enough that I think it serves the same needs as the other companies mentioned above.
And just so we're clear, with White Fox you are hiring them. They contract out the work
, which means that an author might not know who exactly is working on their project. But on the plus side White Fox serves not just authors but also publishers who want to hire a freelancer to write for them.