There are several places online. Just search for Honda atv parts. They will come up. I have ordered from several of them and they were good.
They offer the official parts fiche from Honda. You put in your year and all the details of your machine, and it gives you diagrams which show you what is in there. Very convenient.
I did order Kolpin boxes for my Rancher from Rocky Mountain. They shipped free the next day. But in looking at their website I have not seen the oem parts fiche there. It appears they sell accessories, etc., but that they are not an official Honda parts dealer, as are the others.
I think one I ordered from was Babbitts powersports somewhere in Michigan. Order was just right and I have found they have lower prices than my local dealer, except for the engine oil.
dc