Helpful description. 'I' really helps me. xD JK
You can find a Water Stone from Route 19. You can also find one in Clay Tunnel, however you need a Dowsing Machine.
There are also plenty of places where you can find the Water Stone in a dustcloud. These places are Wellspring Cave, Chargestone Cave, Mistralton Cave, Twist Mountain, Victory Road, Giant Chasm, Seaside Cave, Rebirth Mountain, Clay Road, Underground Ruins, Relic Path, Caselia Sewers, and N's Castle. (Omigosh lots of places :O xD)
If you want to take the easy street, and your rich, you can buy a Water Stone from White Forest, Gear Station, PWT and Join Avenue (from one of the shops that the source never bothered to give a stuff about :3)
Hope I helped. :)