They are becoming easier to find, because these bottles are what you have to put liquids in when you fly on an airplane.
can’t find one anywhere, then you can
just use any plastic bottle with
a spout that squirts, like a Rubbermaid picnic bottle or a kitchen squeeze bottle.
The best, once again, is a small travel squeeze bottle, which is listed as a 4 oz. bottle, but actually holds about 5 oz. or about 140 milliliters. Fill it up from the gallon of saline solution.