I have a gravity fed watering system on my 3 racks that are very much like this one, I buy the solid brass ones so they can't destroy them, I get them off Ebay. They are pretty inexpensive, just search "rat water nipple" and you will get lots of choices

I am trying to "escape proof" my rack, if one more gets out my husband might lose his sanity, lol he hates when they get out......