As it hasn't been mentioned, I thought I would put in a word for the AQWSAFE domestic distiller unit that I recently bought to feed my Fuji Pictrography processor. The £175 investment is soon recouped if your only alternative is to buy bottled distilled water. I'm now using it to provide water for chemical mixing (and the domestic iron!) as well. It's also drinkable of course.
Interestingly, deionised water was too pure for the Fuji Pictrography processor, which relies on being able to send a small current through the water for monitoring purposes.
Details on AQWSAFE here; they also sell distilled water in bottles.