As I said you can buy it through the PlayStation Stores official site that K.wong linked to. You don't need to own any PlayStation device at all. Just log in using your SEN account (your PSN account is a sub account in your SEN account, the user name and password is identical to PSN) and then buy it. The reason you don't need to own a PlayStation is in America Sony is putting PlayStation services like their Pay TV service called View and their streaming game service called Now on Smart TV's and they plan to expand it further.
EDIT: Just logged into the site for the first time in months, looks like they have rebranded the account they use on the site from SEN to PSN. I guess to stop confusion, cause after all, how many people know what the Sony Entertainment Network is.