The obvious way to purchase Microsoft Dynamics CRM Online is via the Office 365 platform and purchase a subscription. The subscription is billed directly by Microsoft on a recurring monthly basis.
An alternative is to purchase a 12 month subscription under Open License, in return you receive a code which is redeemed in the Office 365 admin center.
The part numbers under Open Licence appear to be:
|LX-00012||Online Professional user subscription 12 month|
|LX-00028||Online Professional user subscription 12 month for users that have an Office 365 E3 or Office Business Premium plan or higher|
There are of course many other part numbers for the different license types and add-ons.