Hi well my advice might be abit out of date as this was 10 years ago when DS2 was three months old and we went to spain. I took our own kettle and bought bottled water and boiled the water and made as normal.
You have to be careful with the mineral content of bottled water. Here in Italy they tell you if making up formula to just use boiled tap water. Surely boiling it gets rid of any nasties anyway? Here, if you want to use bottled there is a list of "baby-suitable" bottled waters. Cost an arm and a leg from pharmacies.
I thought part of the argument for boiling bottled water as opposed to tap water abroad was that in some countries the mineral content in the tap water is even higher than the bottled water? Same for adults not drinking tap water abroad - not just because of nasties?
I think as long as you check the sodium content it's ok. This might help http://www.nhsdirect.nhs.uk/articles/article.aspx?articleId=1945
When I go to my mum's in Spain, you can get safe to use bottles of water from the supermarket and it isn't that more expensive than the regular stuff, can't remember what the brand is, but it says it's OK to use for babies bottles on the label.