"Shamanic Abilities and Taboos
Most shamanic abilities and all taboos are always in effect. If a shaman wishes to gain a new ability or taboo, they must contact a Greater Entity. Greater Entities vary from place to place. In Dragon Pass, the Greater Entities are Horned Man, Daka Fal, Waha, Hykim and Mikyh, Kolat, Kyger Litor, Jakaboom, and the Earth Witch." (P 359)
In My Glorantha, I'd allow deals with other spirits as well. E.g., if you want to cross this river safely, the group's shaman has a chat with the local river spirit, who might demand the taboo as payment (possibly unlikely and fairly easy to ignore by the shaman, but I'd see it as possible... )
I'd be inclined to do something similar with Spirit Pacts as well. 1 POW per 10 of the spirit's POW, plus maybe a ban/taboo thrown in while the spirit is still around... If you have a pact with a horse spirit, it might be offensive to (not) eat horse meat while it's looking over your shoulder..
(I'd probably also stress the shaman's Honour passion.. ).
While this is my Glorantha, the rules support it...