Most stores are willing to part with their leftover pallets. Home Depot or Lowes goes through plenty in a season. My main supplier is my local Harbor Freight, which will now let me pull up at the loading dock and take as many as I like w/o asking. If I'm not in that area Agway is just down the road along with another hardware store. Definitely don't buy them!
I use quite a few for my firewood so I probably go through 20 per year. They make good campfire wood after they break... just have to watch out for the nails.