I actually mostly called up HVAC supply places but they wouldn't sell me a cap and offered to refer me to one of their "preferred" service providers. Maybe it's some backdoor colluding in my area, who knows, but yeah I was really surprised they wouldn't. I think I called 3-4 before someone told me Grainger sells 'em.

It's pretty common for outfits like that to be "trade only" and not do retail sales at all. Electrical wholesalers are often the same.