Royal Cauldon hasn't been made since 1962, which is why you can't find it.
Any pieces available today would be second hand. You will need to keep checking estate auctions, yard sales, ebay, etc. and collect it one piece at a time. I checked www.replacements.com, but they have none in stock. If you register with them (there's no charge), they will notify you when a piece becomes available. Ebay sometimes has a piece for sale. Right now there is a chop platter available:
Another source is www.etsy.com but they don't have anything available right now.
I wish I had better news for you, but looking for it piece by piece is the only way to complete your set.