I bought an RO kit from PureWaterClub (now out of business) which came with a 1.5/2 gallon storage tank, which eventually failed. I replaced it with a larger tank linked below that I found on Amazon. The day/night before brew day it takes me around 2/3 storage tanks full to fill my Foundry with 8 gallons. So I spend a few hours waiting for water, but it’s no biggie if you plan for this ahead of time.
iSpring T32M Pressurized Water... https://www.amazon.com/dp/B00439MYYE?ref=ppx_pop_mob_ap_share
iSpring T32M Pressurized Water... https://www.amazon.com/dp/B00439MYYE?ref=ppx_pop_mob_ap_share