In FL 1.30d(newest), first you need to buy the Slayer/Dragoon class, and be of a high enough level to get the ability Ignore Height(more is a good idea, since you're gonna be facing a miniboss).
Then, you go to the harpy nest on top of the southwestern mountains(circle around the Dragoon trainer until you meet some trees and harpies).
South from the harpy nest should be a cut-off relatively large zone of dirty rock tiles. Use Ignore Height to get there.
After that, go south. Meet the Dragon Slayer. Kill him. Get the spear. Profit.
Other one-of-a-kind agility weapons are the Bow and Light Arrows(donate 2.5k to the church before anyone else does), and the Quick Dagger(steal it from the equipment shop's showcase)