Solvents (water) is found in water skins or you will see spots of "bubbling" water in rivers/lakes etc. If you have keen eye for alchemy, the water spots will also smoke. Natural water is the lowest level solvent and makes a level 3 potion. Solvents can be found in the zone equivalent to the level of the potion it makes.. so a level 3 natural water will be found mostly in your starter islands, and likely in the beginning area's of your first zone.