If you have an iPhone 3G or 3GS or a first gen iPod Touch, you can get a copy for free. Apple released iOS 4 and changed the minimum system requirements to Leopard or later, which meant that Tiger users had to upgrade in order to keep using their devices.
I know that this program was still going on a year ago; my guess is that it's still an option. Go to your local Apple Store and ask the geniuses.