Try a drug store.
Heck,I might be tempted to try some mineral oil from a drug store.It is an old fashioned laxative,so you will know where to find it.Since it is pharmaceutical grade, it won't have any additives.The only downside is I suspect it is higher viscosity than the brake mineral oil your disc calls for.It also won't have whatever additives the brake folks use.
I don't think the auto parts guys will have pure mineral oil.
Luck,
Charlie