Buy a bag or 2 of all natural charcoal, along with a bag or 2 of beach (fine) sand . You can make your own reverse osmosis system, with the sand and charcoal, after crushing it. Fairly labor involved.

Or you could buy one. IF you went the buy it way and no one has any on da island. I'd be happy to order here and drop ship it, to you.

Lower budget , minimal plumbing needed
https://www.amazon.com/Countertop-Re...s%2C129&sr=8-7

Counter top. One of the many, but good flow rate
https://www.amazon.com/Waterdrop-Rev...9&sr=8-12&th=1


Then there's the time consuming, plumbing required under counter units.

https://www.amazon.com/iSpring-RCC7A...09&sr=8-6&th=1