I had to buy my replacement Doulton cartridge off ebay and get it shipped over with EPS. I couldn't find a Doulton one here. Ochoa sells water filter kits, but I don't know if they filter down to the 0.9 micron and have the silver bacterial layer that the Doulton ones have.
Make sure you get a genuine Doulton filter. They are the world leaders in water filtration and they do work. We've been drinking filtered water from ours for over four years now and none of us have been sick. Although we have to wash the cartridge every three to four weeks when the flow slows down.
We get our water from a well that supplies our local community. It's a private enterprise run by a Canadian guy, not Coraplata. The water is pumped into my 5,000 gallon cistern which I treat with a chlorine puck every two weeks.