In Montreal, Canada, we get deionised water in pharmacy stores (at the chemists?).
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.
Using DI water instead of DW is like using a cannon to shoot gnats.
The only thing worse (more expensive and uselessly so) would be to use DI DW. That is way overkill.
Use Distilled Water. Here it is less than $1 / gallon. You can even buy small stills to make it yourself.
Alan I too was shocked by the local Halford's price which was exactly as you have quoted. However there is something strange here as others who have recommended de-ionised water on another site where users are predominantly U.K. based have said that it is available in big volumes at quite cheap prices. Unfortunately I cannot recall the stockists mentioned. I had thought it was Halfords but if so I couldn't find any! Maybe a google search will turn up something although ideally it should be available fairly locally. Liquid is expensive when mail ordered.
However if this stuff cannot be obtained cheaply then it might be worth checking your process. I say this as I too live in a very hard water area(the kettle calcifies very quickly) and yet I have never experienced
calcium deposits. I use the Ilford wash sequence and then 1 min in Ilford wetting agent and into an air dryer.
What I think is important is to risk drawing the film between your middle fingers after the wetting agent and then hanging your film up vertically. I have never experienced scratches in so doing. I have a fan dryer which may help. A hairdryer on the cool setting may help if you don't have a drying cabinet.
The other trick is isopropyl alcohol with the wetting agent and giving the film a quick shake. I have never needed this but Les McLean swears by it.
I had always assumed that the NorthWest of England including Cheshire had relatively soft water and is mainly fed from Welsh water which is quite soft. Maybe not but I'd be surpised if the water is harder than in S Northants
Best of luck in your search
You need to use distilled or deionized water only for the last bath, the "final rinse". If your tap water is of normal quality it should be perfectly fine for all other baths. You can reuse your final rinse several times (you are supposed to wash the film very well to remove any residue of fixer, and if you do it you will also avoid contaminating your final rinse so that you can reuse it).
Rome water is quite problematic as far as calcium deposits are concerned, but I never have had any water mark problem as I prepare the last bath with cheap water for steam irons.
So the cost really is no object here. Pick the most convenient provision. Don't buy deionized / distilled water in large cans thinking you are going to save money by buying in quantity (unless you need it for ironing, that is). The opened flask of distilled water can develop a mould. The ideal would be to prepare you final bath with a new flask. You will reuse it for some time. Before using it visually check for solid deposits, mould etc.
Do install some filters on your tap so that you avoid putting some sand / solid particles in your baths.