Originally posted by GunsForBucks:

RNG
Also berzerkers have a chance to drop them as well but it is a rare drop.

Def rare, killed so many and only ever received one as a drop from them.

Totems are either plentiful or scarce. But I would say they are the hardest and most unreliable boss summon item to get at the moment, especially if your looking at resummoning a few times.