Depending on where you're located, there are many many businesses that throw away their old pallets. You will usually see them stacked against a building. Just ask and most will be happy to let you have them.
The great thing about pallet wood is that they're made from all kinds of wood. Although most are pine, they will use whatever scrap they have lying around. I have pulled cherry, redheart, ambroisa maple boards and more from pallets to use for my projects. You just have to look.