I'm told that Apple will still sell you Leopard. It's not a product listed on the Apple Store website. You have to call them. Get the number from the website.
Going from 10.4 to 10.6 is a matter of controversy. Apple wants Tiger users to purchase the Box Set for $169 which includes iWork '09 in it. While it works to purchase the $29 Snow Leopard single license, that's really intended for people who already own Leopard.
You could just buy a new Mac. You'd get SL with it.