You dont have to permanately attach an RO/DI unit, they can also be connected to your sink faucet or the water line for a washing machine etc. As to buying water, I suggest caution, irregardless of what they may claim, I have used water from Hy-Vee etc in the past, it was at best marginally better than tap water, I would see if you can borrow a TDS meter and test it before using it or spending much money on it.
Quite honestly, I doubt you will find DI water out of a machine as this is usually meant for drinking water, you do not want to drink DI water. If it were me, (I know the cost is greater and its a pain, but would get you by), I would use distilled water. JMO
Allen