Nebulous Voidcores

May have already been asked before but I missed it so I apologize.

Is the only way to get the voidcores this season by have the option when completing 3 slots from the vault or can you buy them for the vendor? Don’t have all the time I did last season to get on so most weeks I may only get a chance at 1 or 2 vault slots. They are not available at my vendor at the moment