portable ro filters

are the portable revesrse osmosis filter pretty much composed of the
just the replacement cartridges
strapped together and connected via some fittings etc. i could
purchase at Lowes or home depot?

i was thinking if that be the case it would be smarter to buy a supply
of replacements filters that hase like 6 of each stage for much less
than a single "kit"
and assemble it myself for alot more filtering years worth of value.

i've never owned any of these and its not completly clear from the
photos on ebay so i thought you people might tell me if its that
simple or will i be missing some desperately needed part that cant be
found affordably locally.