As a matter of curiosity, why would one use deionised water for mixing film developers?
I don't think the OP said what he was using this for, but if it is to be used in the mix I would ask the same question. However, dunking the film into deionised water for about 30 seconds after the wash will give you a film free of drying marks. I always do it. You can get this from any branch of Tesco/Morrison/Halfords.