Best Buy sometimes has them on sale for a decent price as well. We have purchased the ones my kids use there and they were both free w/rebates. Also, if you do find them free...get two, as the batteries don't seem to last very long after you've been using the phone awhile and it's a lot cheaper to just start using the other phone instead of trying to replace the battery.
I have to say that this is the perfect phone to use for limited calls/emergencies as they offer lots of special deals to top-up once you've activated the phone. It works out to be about $5/mo. for each of my kids.