Googling "plants vs zombies soundtrack", the first result led to Laura Shigihara's Plants vs. Zombies soundtrack. It has the whole set, as Shigihara is the composer. There are also some tracks which are not in the game.
You can listen to the individual songs right there on the webpage. If you want to download them, you have to pay, though. But I like the site.
A few of my favorite Plants vs. Zombies songs-
And really- who doesn't like this one?