I know this topic is old as dirt, but I too was/am looking for 1/4" OD rigid tubing. I stick it down into my top off reservoir and my cal/alk dosing jugs. Regular RO tubing works, but it does coil up a bit. Even if you heat it a little and try to bend it in the opposite direction, it still wants to coil up. It's hard to get the end of the tube to stay at the bottom of the jugs.
Anyway, just thought I'd mention that Lee's Pet Products 3/16" rigid tubing slides right into a small piece of RO tubing, which then can connect to
quick disconnects, etc or what have ya. Essentially just using an inch long RO tube as an adapter. It's small stuff, but it works.