First up, this is only really suitable for long stays (at least 2+ weeks).
If big 20 litre "water cooler" style water bottles are available for sale somewhere, chances are there's some kind of refill service available. I've found an article by UNICEF that talks about one such company it has helped to get started, and the prices look like what you're looking for. The way UNICEF talk about the company makes it sound like the service isn't unusual, and that this particular company is remarkable simply for bringing this service to a remote community:
The system works by collecting water from a local arsenic-free well and pumping it into two large tanks where a series of filters remove large pieces of dirt and excess iron to ensure it will have a good taste.
At the next stage the water is pumped through a UV (ultra-violet light) treatment system which takes out small particles of dirt and kills bacteria.
...Currently each new water bottle is sold at the low price of 18,000 riels (US$4.5). A refill costs 1,200 riels (US30₵). They also supply the local primary schools with 20 bottles per day, free of charge.
The specific company they talk about is probably not in the right location for you, but it's an indication of the sort of thing to look for, a setup based on:
- A small stockpile of these bottles
- Weekly (?) refills through a water company
- Some kind of pump if you don't have a water cooler, almost certainly available somewhere "home plasticware" is sold, like this:
How to find such a water company is the tricky part. Some suggestions:
- Look for contact details on the bottles themselves
- Ask the people who sell the bottles (who may well act as middlemen for a small commission)
- Find a local business directory
- Ask around, particularly smart office buildings of the type that likely refill water coolers for their staff. Local NGO branch offices probably are a good easy place to start.
- Ask relatively well-off locals what they do!