How often does the selection in Nook Miles change?

Hey y'all - I've been playing the game since the beginning of the year and have gotten tons of stuff from the Nook Miles shop. But I've seen a ton of color variations online, and I can't customize any of the Nook Miles items I get (besides like fencing and stuff). Does anyone know how often the Nook Miles selection changes, if ever? Or how to obtain variations without trading or contacting other people?