That totally depends on the world you are playing. And in general it is like with any other thing on the free market - demand and availability dictate the price.
If you play on desktop just ask in the global chat for an offer.
If you only play mobile, I would advice you to go through the official forums. I am not sure how the forums of other markets are organized, but usually there should be a sub-forum dedicated for every world. In that one you may find players offering goods or you can place your own request for an offer.
If you do not like the forums, you can only ask around in your guild/friends list/neighborhood, if they can advice you with the name of a "trader".