Yesterday Kalazz noted that the PCs used up all of the unholy water needed to get into the tomb of Sakatha in the Cold Fens.
He's absolutely right - the last delve was a real last delve.
But it's not like there is no unholy water in the entire world.
But equally, you can't go to the local Demonic Supply Store and buy some. And what you buy from some back-alley vendor is likely to be fake.
So where do they get it?
Pretty simple - they'll find it in play.
I suppose they could make it, too, if any of them were an Evil Cleric and worshipped Sakatha. But evil worshippers of a demonic religion don't generally make good companions for people wanting to kill and loot a tomb associated with it.
But instead it's a question of finding it.
Research skills will tell them about associated evil religions, in case "close enough" is good enough for getting past such doors. Maybe it doesn't need to be consecrated to Sakatha, maybe it only needs to be evil and demonic.
Current Events skill would help find news of wild cults that would need to be hunted down and looted for their unholy water.
Simple, old-fashioned delving might turn up more evil temples and shrines that could be looted for such liquid.
Returning to temples found and looted might turn something up - but this is perishable stuff, and usually the first thing spilled out when PCs tear through an evil cult.
Purchasing won't happen, but if a supply is found, and identified, and then preserved, the PCs can make a quick delve or two into Sakatha's lair. The current delvers are better equipped and know what they'd be getting into.
We will see how they manage!