The Dream had a Tiffany & Co shop right on the ship a few years back but it didn't last too long. It seems that Dream and Fantasy landed on Carriage Jewelers (not a brand just a Disney-owned shop), and they sell high end/luxury jewelry, but I can't say for certain if they still carry Tiffany
The Magic & Wonder has their jewelry in White Caps, while the Wish has it in Enchanted Castle Jewels.
They USED to sell a few pendants through the Onboard Gifts website that you could have delivered to your room before you boarded but those haven't been sold for a few years either.
So... depends on the ship. There's certainly a place to buy them on each, but again I can't say with certainty if Tiffany & Co. are still being carried.