Normally, I'd suggest employing the skill Use Magic Device to turn purchased scrolls of curse water into wands of curse water, but that doesn't seem an option here. Thus, with scrolls and wands of the curse water spell unavailable, and the adventure path seeming to expect someone in the party to be able to manufacture a monstrous volume of unholy water—so much that it would normally require over 1½ tons of powdered silver—, it seems that, unless the GM interferes, Team Protagonist is stuck in the ugly spot of trying to create an original magic item: a wondrous item that casts the spell curse water.
If neither the storm druid nor the wizard has taken the feat Craft Wondrous Item, the one with the higher Spellcraft skill modifier can, if the rules are used in the campaign, follow the rules for retraining and take a few day and spend some gp to swap another feat for the feat Craft Wondrous Item.
Then the party member with the feat conducts whatever research the GM deems necessary for creating an original magic item—none, a few days in contemplation, a week at big library, whatever—, and the GM reveals the magic item's price. The Magic Item Creation section's Table: Estimating Magic Item Gold Piece Values provides guidelines that indicate a wondrous item that can upon command create an effect like the spell curse water has an estimated base price of 4,300 gp (1 [spell level] × 1,800 [base price]) + (25 [material components cost] × 100 [for unlimited use]); if can only be used thrice per day this price drops to 2,580 gp. (This GM would likely round these both up to at least 5,000 gp and 3,000 gp, respectively.)
Afterward, the PC need only get to work on making the party's wondrous item, with the primary creator increasing the Spellcraft skill check necessary to complete the item by +5 because—obviously—no one in the party and no one they know can help meet the item's curse water spell prerequisite.
Note: This GM believes it a terrible oversight by the adventure path's writer to assume that among the PCs will be one capable of casting the curse water spell. Further, even having the PCs take up to two weeks to create a 3,000 gp workaround because the writer lacked forethought seems almost too laborious. If you think so, too, this GM—who, to be clear, has neither read nor run this adventure path but based on this question alone—recommends that either the gp for the 3/day item fall into the PCs hands or the PCs discover a map to secret evil survivalist's cache of unholy water.