Charlieo, I haven't used the Rain Water brand water, but the description in the link you provided looks as though they've processed through a number of good filtering steps (including both absolute filtration and reverse osmosis filtration) that should create a good water to use in your cleaning regimen. Justin_time's post is considered by many here to be pretty well definitive, so I'd be confident in his recommendation.
The NERL Diagnostics Reagent Grade water is perhaps one of the best waters available for the rinse step (which is by far the most critical, and benefits the end results most, in the use of an ultra pure water). As you point out, the cost is high and buying in 1 gallon containers presents some challenge if one will not use it within a reasonable time after opening. Nevertheless, purchasing the 9800-1 which contains four 1-gallon containers does get the cost down to about $30 per gallon. (This is where fellow vinyl nuts living nearby come in very handy. :-) )
.
The NERL Diagnostics Reagent Grade water is perhaps one of the best waters available for the rinse step (which is by far the most critical, and benefits the end results most, in the use of an ultra pure water). As you point out, the cost is high and buying in 1 gallon containers presents some challenge if one will not use it within a reasonable time after opening. Nevertheless, purchasing the 9800-1 which contains four 1-gallon containers does get the cost down to about $30 per gallon. (This is where fellow vinyl nuts living nearby come in very handy. :-) )
.