Actually, if you don't mind Lawbreakers, sacrifices can be stored in a few ways;
1) Kill the victim ritualistically in the first place and take symbolic trophies - no need to hoard up live victims when you can offer their torn-off hearts to your patron.
2) Transform target into a miniature statue of themselves (or a coin!) with a Transformation/Disruption spell then dismiss the transformation when you need them back.
3) Transmigrate the target's soul into another, more easily carried living being or lifelink a human with such a being . Rabbits, cats and other pets are all good options for the aspiring necromancer. When the time comes, kill the rabbit/other pet thus also killing the human without needing access to the original body.
4) Entrap the target's mind into a mirror or voodoo doll, leaving their body comatose, sleeping or delirious behind and make sure that destroying the object also kills the victim. Leave behind as many "sick" people as you want for the mortal authorities to take to hospitals and take care of them for you. You can slay them at the time of the ritual through the mental link.
5) Hit the victim with a mind-mojo to make them willingly follow you around. No need to hold them captive or even hide them till the sacrifice. You could even program them to kill themselves at the specified time, saving you the trouble.
6) Cast a temporal banishment spell on the sacrificial targets that sends them directly to the site of sacrifice at the correct time. You could amass victims over months with such magic and still have them appear where you want them (i.e. over the sacrificial pit full of iron spikes) when the time comes.
7) Make a tiny weakening of the Veil to the Outside so that you create a sort-of wrinkle, a dimensional bubble of sorts that can store someone for a time. Store your captives in this bag of holding thingy. Kill them when the time comes. Alternatively, offer up the victims to an Outsider immediately with this deal in mind; the Outsider takes their souls and when it's time for your ritual, they sponsor you for half that much power, keeping the difference for their own use.
See? At least one way to store sacrifices per Law broken. Still, an Item of Power storing the energy would make it too easy - and far less interesting.