The instruments in K-On have labels with real music companies. For instance, here's Ritsu's yellow drum set (I'm not quite sure where this picture is from, or even if it's just fanart, but the drums are clearly the same as those in the anime):
That's a very clear Yamaha logo. Either Yamaha sponsored it, in which case they'd want a real product to be used, or the artist chose to use that particular model, but either way it's very likely a real-world instrument.
What models are their instruments? As a note, I think a couple of them might have multiple instruments, since I've seen Ritsu with a different drum set also.