Minecraft wiki entry for Curing_a_zombie_villager, associated text: To brew a Splash Potion of Weakness, you need:
1 fermented spider eye, which can be crafted with spider eyes, brown mushroom and sugar
1 to 3 water bottles, which can be obtained by right-clicking a water source with glass bottles
1 gunpowder, which can be obtained from creepers and ghasts or found in naturally-generated chests
1 blaze powder to fuel the brewing stand