This depends on what your situation is. Do you need someone who can immediately begin teaching your team how to improve at the speciality, or do you need someone who will take the share in the work with the rest of the team?
In the first, what you’re looking for is an existing domain expert. In the second, you have the option of training someone to become that domain expert. The second is much easier to do.
If you’re hiring for the expert-level role, look at other companies with a similar role and see what you can do about enticing one of them to join your team. You also have the option of hiring a consultant (who tend to focus more on specializations than FTEs do) here.
If you’re in the second scenario, reframe your thinking from, “Does this person have these specific skills?” to “Does this person have the capability to learn these skills?” With an existing, capable team around them, you can create your own domain experts and that’s so much easier (and less expensive) than hiring existing experts.