Here is where you go to start your purchase of Dynamics CRM. http://www.microsoft.com/en-us/dynamics/crm-purchase-online.aspx. It will also give you info about the Sales Productivity Promotion. During the sign up you should have the chance to enter in the number of Pro, basic and Essential users you need.
For reference to what license version you need please see page 8 of this PDF, http://aka.ms/gtn5wt.