There is no organised trade in magical artifacts. What shopkeeper would be foolish or powerful enough to keep more than a few such artifacts in the one place? It would be like a signal-fire, lit to attract wicked sorcerers, demons and worse things from all over the universe, if not beyond.
It is said that one does not look for magic items; rather, they look for owners. Whether this is mere superstition, certainly such objects are rare. One who goes forth expecting to purchase, for example, the Remarkable Mittens Puissant Against the Unquiet Dead, is like one who goes forth saying to oneself
"I hope I meet my true love today; but if she be named Annie or Alice I shall reject her, for I rather hoped for a Mary." Yet the meanest bazaars have a way of selling remarkable items; just as one may find love in the least expected places, and if it be fate or mere chance who can say?