Unless I'm missing something, I'm not convinced the TOPS whistle is unique.

I have quite a few (apparently identical) flat three-chambered whistles and they've come from a variety of sources, including some with boating safety or advertising messages. They're generally referred to as marine safety whistles. Boating stores/departments sometimes carry them. Ebay sellers have them too.

The flat whistles I have aren't necessarily the loudest (my big Storms easily blow them away, as do the new Fox whistles).

But they're handy in jacket and pants pockets. And most importantly, my dogs respond to them better than anything else I've tried.