Tokullectibles.com is where I got it. Note that this is the Japanese release I bought so it was a tad more pricey than the global release, but it will be the exact same product regardless. The global release will also have an English manual!
It's resources for performing specials with Death. You start with 13 (akin to the mobile game). Each special has costs. Death taking a hit also lowers a stock. When all stocks are out, Death can't be summoned but Nier has a 20 second cooldown before stocks replenish.
In addition, one of her Skybound skills removes stocks all together and becomes a gauge that slow drains overtime, granting many uses of Death while active.
