Opposite of "client"?

Solution 1:

You have pointed out some of the difficulties related to the broad range of possible categories of services involved. Lawyers have clients, as do carpenters, manicurists and numerous other professions and tradespeople.

A term that is sometimes used is service provider.

BUSINESS a company that provides a service, for example insurance or medical treatment

It distinguishes the company or person proving services from a provider of goods (whose counterparts are usually called customers). Note, however, that the term is also used to identify a company that provides internet connectivity.

Solution 2:

You might consider:

Consultant: 2: one who gives professional advice or services : expert

from m-w.com