If the existing crush washer is ok and not leaking, you can just reuse it. Most people do. Or find the part # at ronayers.com and go to the Honda dealership and buy a whole bunch of them (if you want to use a new one with each oil change). That's what I do. They're all of a few pennies each, so cheap to buy in bulk from Honda.
I bought a handful online when I was getting other parts, but every dealer should have them. I think they are pretty much the same for all bikes, so you should be able to find them at any motorcycle dealership. I'd stay away from Ducati and BMW dealerships though, they may charge you 3x the money for the same thing...