I have millions of them. All descended from the original one, of course. They are one of the niftiest of pets, I think, so I morph them a lot. Let me know if you want to meet in the morphing tent this weekend and we can try morphing you from one of mine.
The Quackosaurus is dropped by pretty much any boss in Skull Island. While I was farming for the Orchid Chameleon I ended up with two Quackosaurus, one from Fin Dorsal in Gullet, and one from Joey Karo in Blood Shoals. I got both in about fifty battles, which for level 65 doesn't take too long.