Where do you get u-turn in Pokemon pearl diamond and platinum?

The TM for U-Turn can be found by Surfing around Canalave.

You can also buy it at the Veilstone casino.

Like the person before me explained, U-turn can be found in Canalave.

Just go onto the dock where the sailor is waiting for you to go to Iron island. Face south and surf for a little bit until you get to a small piece of land that is surrounded by trees. Explore around there for a bit and the pokeball should be easy to find.

TM89 can be bought in the Veilstone Game Corner Prize building for 6000 coins.