Buying bottled water

I’ve been visiting Iceland for over 10 years and have enjoyed the tap water. However, my next trip will be my first while immuno-suppressed and having checked with MAST and my doctors I’ve been advised to rely on bottled water in rural areas. I think the risk of infection is very small but in my specific situation if I do become infected it will be either difficult or impossible to treat.

Having never had the need to buy bottled water, we finally get to my question: Are large containers of water generally available in grocery stores? It’s a three week trip spent mostly in rural areas and the Highlands and I’d really like to avoid buying a very large number of small plastic bottles if possible.