hardwhear store its a shower drain cover or a pool shop spar shop .
Home Depot has them in chrome and brass . I use them for guitars and amps. Needs a 1 1/2" hole.
Hardware store huh? Well I suppose that's one way to get them. Or do what I do and travel with a screwdriver and liberate them from hotel rooms you stay in. :D
As has-been mentioned in these forums before, referring to Home Depot, Lowe's, Ace Hardware, etc. as musical instrument supply stores is the natural outcome of this obsession.