I used a Culligan tank (mixed-bed deionizer) to clean water to be electrically pure for high voltage laser pump tubes. We used the stuff for other purposes and if you want clean water, the deionized water works great
The stuff is like a mild acid (no it wont hurt anything) and will “scavenge� the deposits off windows and paint.
We got our setup from Culligan -- we ran a closed-loop system, so a pretty big tank (about 5 foot in height) lasted quite a while for our use (we used the water for G-jobs and for our equipment). We just paid them a monthly fee; it was around $10 month for our needs.
You might just try calling them to see what their charges are for comparison.
The "resin bed" is only good for xxx-cycles and will need a recharge.
As to who is the best "portable" maker of "deionizers", I don't know. However, I did like the results I got from the mixed-bed technology and needed water that was basically non-conductive (dead pure).
You might want to see what the "upfront" costs are vs. recharge costs to compare different technologies. (Think about this like color ink-jet printers: some are rather cheap, but the supplies are rather expensive and over a year a unit with a low "upfront cost" can really kick you in your wallet compared to a more expensive unit with cheaper consumables)...
LINK to what I was using:
http://www.culliganmiami.com/pf5.html
"In a mixed-bed deionizer, cation and anion resins are thoroughly mixed in a single tank. The mixed resins act like a series of alternating cation and anion exchange tanks to produce very high quality water. A mixed-bed deionizer typically produces water with greater than 10,000,000 Ohm-cm resistance, which is equivalent to less than 0.05 mg/L of sodium chloride..."
They would just swap tanks when the water quality lamp would light (it's a small neon bulb).
http://www.culliganmiami.com/pf5.html
http://www.culliganmiami.com/page5.html
YMMV