Vendors never sell items that are considered "unique." You can only get unique items as drops in the dungeon. For most unique items, they are only available as random drops, as opposed to a guaranteed item from a particular quest or enemy. Only enemies of a sufficient monster level ("mlvl") can drop a unique of a given quality level ("qlvl"). According to Jarulf's Guide, Windforce is a qlvl 17 item based off a Long War Bow, so you need an enemy that can drop LWBs. In this case, that does not change the problem. Certain very rare unique items are rare because of the combination of base type and unique item qlvl. The Helm of Sprits [sic: the name is misspelled in the game data] is such an item.
If you want to find the item in the minimum amount of time, focus on killing boss monsters, which have a higher chance of dropping a unique item. Normal monsters can drop unique items, but rarely. You can further optimize time by focusing on bosses guaranteed to appear, such as Lazarus + Red Vex + Black Jade, which are present in every multiplayer game, in roughly the same area of level 15. Teleport quickly through levels 13 and 14, killing only what you must to avoid being slowed down or overwhelmed. Kill any bosses you find along the way, but don't spend your time searching the level for them. When you have killed Lazarus, Red Vex, and Black Jade, if none of them dropped what you want, leave the game without returning to town. (This assumes you left nothing in town that you want to keep.) When you start your next game, visit the Witch and the Healer to restock, then repeat the run.