Your best gains will come when your skill is within + or - 25 of the creatures barding difficulty. This is a very good guide for non aggressive from Poo. It works great for both discord and provo.
0-33 buy from the GM NPC Guildmaster.
33-65 polar bears
65-87 gamans
87-120 (if ya want) cu sidhe
(hiryus are better for top end gains)