I actually bought them from my water supplier, but saw them in hardware shops as well as in Hypermart, but that's a few years ago, so I might be a bad reference...
I think Rhoody is correct, but there is also a plastic container shop that sells all kinds of water containers along with rice sacks on the right hand side right before you go over the bridge going into downtown Dumaguete on the same road as Citi Hardware.
