Ghost's Keys are contained within "Sealed key of soul", dropped by the Soul Ruler found after the two trap hallways in Legend 2-9. The fastest way to get keys is to quickly rush through without killing anything other than the skeletons at the beginning, kill the Soul Ruler, and grab the key if it drops. Unfortunately, it isn't a 100% drop rate, but it is fairly common and you usually won't have to do more than 2 runs to get 1 key.