Thank you! I ended up ordering Wabco Type "E" horns, as those looked more or less similar to what appears in photos, and were available for tolerable cost (mail included) too.
One of my UP PAs has one air horn missing, and the replacement part from the manufacturer is not available, so I thought I would look up what horn was used for the UP PAs, but I could not find a conclusive answer; Nathan KS-1 (2?) or Leslie A200 look more or less like the ones I have, but it would be nice to have a reliable source for an unequivocal answer?