Turtle bone and cuttlebone is the exact same thing. Turtle bone is just marketed to turtle owners and they charge more for a smaller piece. I get my cuttlebone from Wal-mart; a 12 inch box for about 88 cents.
I have had both, I notice that the turtle bones are of higher quality. They are very thick as well. Much thicker it seems than the average bird cuttle. The turtle bones are 6" long.