The local Tescos/etc do not stock any.
Just to clarify, Yes if I had a cheap source (as I used to do when I worked in the analytical industry), I would use deionised (or actually I used to use double distilled,double deionised water but that was because I used gallons of the stuff every day!).
Distilled only removes the organic compounds and not the inorganic contaminants. Deionised removes (reduces unless a VERY good setup) the INorganic bits (calcium/sodium, etc.).
With our water,it is common even with a photoflo in the final rinse to get white marks left on the film.
Also as I have already said, before I had it to hand, so it was just 'better practice' to use so I did.