According to the latest CU:

“But what if I want to create a party that is made up entirely of giant druidic badgers?”

We hear you loud and clear, community. Should you so choose, you’ll be able to create a custom party without any Origins, opting instead to have Hirelings join you on the road – mercenaries who can be hired to join your team temporarily.

In Baldur’s Gate 3, there are 12 Hirelings available for you to recruit – one for each class. Each has their own name, visuals, and race, and can be respecced at any time.

From what it said elsewhere in the CU it looks like Withers is the respec guy. I didn't see any mention of where you hire the hirelings.