The Permanent Transient is a private club dealing in magical items for its members. It is curated by three Treants who are collectively Sumat Pol, each from a different timeline or dimension in the Timescape.
The address is 314 Iter Borsa
It's in Blue Hill (The Blues). In the Neighbourood of the Theater
314 is a reference to the number pi, and 'iter borsa' is Latin for "Bag Process". In reference to the Mathematical consideration of the pocket dimensions in Matt's diegesis (Manifolds).
Behind the Scenes
Both the name of the establishment and the Treants, are a reference to Matthew Mercer's popular creations. The Invulnerable Vagrant, and the Firbolg NPC who has several clones of himself as well, Pumat Sol.