I don't know about Casita, but some of the Yahoo Scampers, who have built things like experimental aircraft, have pronounced the Scamp
rivets as top-quality items (I would presume Casita, having started as a split-off from Scamp
has followed suit in this regard, but who knows?). Off-the-shelf rivets may not be as good -- There are rivets and then there are rivets...
Were I you, I would be inclined to get Casita's selection for both quality and size.
Should be lots of info in the archives about 'replace rivet cap'.