Oh. Then, that's easy! People keep blood on hand all the time for medical reasons already. Plenty of people will donate for free but you could probably have your pick of donors if ya paid 'em for it, then ya just save it for later.

[ he grins, clearly not at all bothered by any of this. ]

Servants need a supply of mana to remain in the world. Usually, when a Master summons one, that's part of the contract that binds 'em together; the Servant draws the mana they need from the mage who summoned 'em. Servants are sorta like familiars, in that way. I've never needed to, but I've encountered Servants who lost their Masters and didn't wanna leave. It can be excitin', bein' somethin' like alive in the world again. Some cling too hard. They'll resort t' anything to survive, includin' what energy there is to be found in blood, among other things. Found out there's some alternatives t' findin' someone t' tap a vein on the spot, anyway.