OK, I see what you're asking, now.
I don't think I can offer a completely direct answer, but...
Is "UPG" the brand of breaker? I'm not familiar with that brand. Usually, though, on the side of the breaker it gives the manufacturer information. That would be one avenue - call the manufacturer and see if they sell directly or can direct you to a retailer. Google, of course, can help. While typing this response I googled UPG Breaker and got this:
https://www.newark.com/airpax/upg6-1-72-253-01/circuit-protector-hydromagnetic/dp/91F9490 Again, not a direct answer, but looks like the brand is Sensata Airpax. Still not one I'm familiar with - but Blue Seas and Sierra are very popular, maybe even ColeHersee - that would be another avenue to find a replacement - if it fits in.
An easy avenue, of course, would be to just call your Grady dealer. You might pay a few bucks more, but of course you're paying for the service and "easiness" of the transaction.