Is there an option to buy wildfire without having a king's landing?
None that I know of unless you activate cheatmenu.
Wildfire is a bit underpowered. The PC can use it to some effect, but companions can't hit the side of a barn with the stuff. It's fucking ridiculous really. Huge Area Effect, high Horse Archery, and they literally throw it 50 feet from the target.
Also, it would be cool if I could compel them to throw it at the start of a battle while troops are massed rather than at the end when there are only a handful of enemies left.
Sigh.
Better off giving them a bow.