Edit: Minecraft 1.0 has added the ability to turn Pumpkins into four Pumpkin Seeds each, by placing it into the crafting grid.
Currently, the only way to obtain Pumpkin Seeds is to find them in Abandoned Mine Shafts; you can't craft them like you can Melon Seeds. This means they aren't a renewable resource like Melons are. (I think this was intentional by Notch to keep Pumpkins a rare item.)
In 1.9, you can just stick a pumpkin into the crafting grid and make pumpkin seeds!
Now in the more recent updates you can get pumpkins in open plains, mine shafts, villages, And many more places. But the most common are the pumpkins in the plains.
In your world, pumpkins can spawn randomly. So if you find those pumpkin, you can change them into pumpkin seeds in 1.5.2.