I usually have better luck finding these sorts of things at Lowes (rather then Home Depot). Look for 1/8" plastic pipe nipple and/or go through the various small plastic fittings in the plumbing section. Check with one of the water treatment places too. Check with Omni life, have them send you a new one.
Alternatively, just firmly wedge some airline into the hole, forget the fitting.
Edited by jfinch