Hold Portal potion

In OSR games it can be very advantageous for thieves and adventurers in general, but mostly thieves, to have a potion of "Hold Portal". The idea, being that the thief can be making it away with his ill-gotten gains, dodge through a doorway, splash the doorway with the potion and seal it against any that pursue.

It would also be handy for sealing pockets, panniers, and trunks I suspect. Not to mention closing that bottle of wine against your friend that's drunk too much already.

So, how do you get the potions?

The thieves guild sometimes have them for sale, 2 in 6 for availability, and then a cost of 150gp.

Magical stores have them on a 3 in 6, and charge 200gp.