The cheapest way long-term is definitely to get yourself a RO/DI filter, fit it at home and use it to produce water like this. Buying it from the LFS or local store doesn't make any long-term sense, though it may be initially cheaper than buying the RO/DI unit.
You don't have to buy a really expensive unit either. I'm planning to buy a Premium Aquatics Supply unit - good value and probably as effective as any hobbiest will need. Buckeye Field Supply is also known for selling good value units, though I've personally got no experience of them.
What sort of aquarium are you planning? The amount you should spend (RO vs RO/DI, 3 stage vs 4 stage etc) might depend on what you're planning to keep. Are you planning to keep coral? If so, are you planning to keep SPS?
I am not trying to keep an SPS tank. I would like to keep to keep coral tank with some fish in it in a 75 gallon tank. I was told that it would be better to keep the phosphate levels as low as possible. There is a source of water in my building that is RO but it is not DI according to standard phosphate tests.
RO by itself could still contain phosphates, silicates and nitrates just to name a few. While RO only does 90 to 98% of the work, it takes the additional DI process to make it reef quality. Consider RO the pretreatment step for Deionization.
Stick with someone like Spectrapure, Buckeye Field Supply, PurelyH2o or others who are primarily RO and RO/DI vendors or manufacturers and not resellers of everything pet or aquatic, the former tend to know what they are doing more than the latter.
