I've been getting them for free from my local feed store. I've also noticed them stacked up near hardware stores, big box stores, etc. Keep your eyes open and go in and ask when you see them stacked up outside of a business. Also, Intstructables.com has good info on dismantleing pallets.
I am using them to create my duck pen. The feed store told me they used to get a credit back if they recycled but they don't any more so he said to take all that I wanted. I am only going one pallet high but if I need to I can put netting over the top.