The instructor mat only turns up when the lot is designated a Spa or Gym and that set's items are at the bottom of the search list as Spa Items when that function is chosen for the lot.
@joRN1414 The Instructor doesn't turn up for me unless the lot is designated a spa or gym even though the instructor mat is put there. I've just tried it as a room on a residential lot in Newcrest. Sims can do yoga there but no NPC turns up.
I built a Spa in Sulani using Maxis spa rooms as a base, plus a bedroom (you could possibly add more) and kitchen diner and my Sim could practically live there and need not go back to their own lot for several days. All the Spa interactions were there plus a deck, bar and swimming. Put in loads of those Spa lockers and toilets and loads of Sims turn up and walk about in towels. I think the Spa comes into it's own in Sulani.
It was more like a luxurious hotel for my Sim.
Someone might know of a mod. I don't use them myself.