My bottle has a nylon tube that is the same as tubing supplied on cars for the water to windscreen washers so maybe ask your local auto parts store and take a sample. I case there are different sizes the outside diameter is the important one to be exact for you seal when pushed into the inlet of the 'enots' inlet valve on your loco.
How does the silicone rubber tube seal to the Regner water top off valve?
ALL of the rubber tube bottles I've seen or used have specific brass end fittings attached to fit the specific Goodall Valve used, Accucraft, WeeBee, Train Dept., etc.
Regner is just a hard side tube that presses into the water fill up fitting.
On older steam engines some of my friends have, the water line is simply pressed into the goodall valve. The top of the valve has a depression machined into it to give a positive alignment for this. To keep from having a metal to metal contact, which would make a very bad seal, a very short piece of tubing was put on the end of the water line. This tubing sealed the water line to the valve.