If you are looking for a way to connect you water supply for your RO filter ,go to any HD ,lowes or plumbing supply ,
ask them for a speedy supply tee .This wil allow you ,to tee off your fliter and faucet under the sink
its 3/8 union on the bottom (this hooks up to your vavle)
1/4" threaded in the middle ,(youll need a plastic ferrel for this)