Use boiled tap water - but left to cool down, obviously, before using if you're worried, but it won't kill the rhododendron to have a bit of ordinary water from time to time anyway - in fact, you could water it with tapwater all the time and give it a shot of sequestrene iron tonic intermittently, that'd solve the problem.

I agree with Bamboo

Here in the Arizona desert, our tap water is very hard, and we have less than 18 cm of rain a year, but we still grow acid loving plants such as gardenias and azaleas. We make sure that the pot has excellent drainage, use a high quality potting compost, water enough at a time to let at least one quarter of the water we applied to drain off, use an acid-forming fertilizer, and add about a ml of white vinegar for every liter of fertilizer solution we use. Sequestrene isn't usually necessary that way, as long as the fertilizer has micronutrients. If you have the funds, a reverse osmosis water purifier can also be useful on the houseplants, and to give the rhodies an occasional treat. : )