How do I get Tome of Polymorph: Turtle since Cataclysm?

As of Cataclysm, you can now acquire Tome of Polymorph: Turtle by fishing. It's a random reward from any school of fish in the new level 80+ Cataclysm zones (Mount Hyjal, Deepholm, Twilight Highlands, and Uldum).

As of today? You buy it on the auction house.

As of today, "Other Means" has not been elaborated upon.

Looking forward, I'd keep an eye on The Amazing Zanzo in Dalaran. With the removal of the old Polymorph Pig quest in Azshara, he now teaches that spell, and I wouldn't be surprised to see Turtle added to him, if it's not added as a rare drop from some zone or boss.