While Creative don't appear to have a dedicated site of their own for purchasing/downloading music, they do state the following on their website (in the blurb on the Zen V+)
Full access to online music stores
Select from more than a million songs from a myriad of online music stores - Napster To Go, Yahoo! Music Unlimited, Rhapsody To Go, and so much more. With PlaysForSure support, you will always be able to play any digital music purchased or subscribed to.
I've got a 4GB Zen V+ myself (which includes a radio) and am very happy with it.