It's called vacuum pump oil. If you go to an air conditioning supply house they'll have it. When you store it, make sure you keep it capped tightly. You don't want moisture to get into the oil - if it does, the pump will not pull as good a vacuum. That's usually the reason you change it in the pump.
The reason A/C places carry this is because when you break into a freon line (meaning cut the line for some reason, such as installing a new condenser unit), you have to evacuate the line before putting new freon in.
Mike