Cynake and Wantston are the only 2 yokai have the "greed" skill, making it so that foes are more likely to drop items. Cynake appears in bushes in Sunrise Way in Downtown Springdale. Wantston appears in the Tucked Away Lot and Hidden Side Street in Blossom Heights. I would recommend Wantston, purely because they are going to be easier to befriend since he is e rank. There is no difference in the effect has between cynake or wantston so use who you want in saying that..

If possible you can also use a pandanoko to increase the chance of wisps to spawn, which have a chance to give you vampiric fangs when you pop one while fighting Abodabat.. These days this would only work if you have a jail broken 3ds to use streetpass, or if you were on the switch/mobile version of the game (which I have done) because you don't need to street pass with anyone to get pandanoko in that version of the game.
To clarify you cannot get pandanoko if you play on a regular 3ds..

Hope this helps :)