Portal wands. It is well known they are made by clerics of Mystra, and distributed to various places on the Amian isles and even Ruathym. They are available in any place with a Mystran cleric and a mythal forge. They are also in stock in Cordor, Winya Ravana, and any other starter location.
As for how? There is a certain amount of unfortunate ambiguity. Clerics of Mystra are charged with spreading these devices, and see it as part of their sacred duty (magic for the masses!). So adventuring sorts might get one for a donation to the clerics towards mythal forge operation. Or perhaps they purchase one on their arrival from a distributor.
Until we have actual quests where one acquires a portal rod (something I've wanted to see for a long time), you are free to say how your character acquired one. Clerics of Mystra would provide with the Lady of Magic's blessings, purchasing one from a distributor is legit, begging at the Commonwealth center as Silky suggested is also okay. If you are uncertain if your idea fits with established lore, by all means ask!
