There are two types of NPCs you can get this item from:
1. Stronghold trade merchants. They're appearing randomly in your and other player's strongholds. With some luck you can catch them there. Don't forget the strongholds of other players! You can visit them anytime.

2. Trade ships. These are also appearing randomly in the sea. Near the ports and islands. Sometimes you encounter them while sailing, sometimes people report them in the global chat. Ships are a little harder to catch, because they have limited (about 30 min long) trade period after which they'll sail to another place or disappear.

And one more note. All of those timed traders in the game are not selling their entire inventory all at once. Some items are there always, but some are appearing randomly. For example, if a merchant sells some ship upgrade parts, you will see only one or two of such items being sold at once, even though there are more items of such type in his sell list.

This adds another degree of randomness to the game, so you should be always on look out of such NPCs, when you're visiting a stronghold or sailing.