Hi,
You should be able to use link below to purchase Microsoft Entra ID P1 (aka Azure Active Directory Premium P1):
https://admin.microsoft.com/Adminportal/Home?trysignin=0#/catalog/offer-details/azure-active-directory-premium-p1/AE7A7A7D-7AE8-452C-B496-1E19CF6449C3
The sign-in you use should be billing account owner. Alternatively you may purchase Microsoft Entra ID P2 licenses by clicking on the Details button under Microsoft Entra ID P2 on above page. There are other potential options you may consider such as Enterprise Mobility and Security (EMS) E3 or E5. EMS E3 includes Entra ID P1 and EMS E5 includes Entra ID P2.
Entra ID is licensed per user per month.
Buy or remove licenses for a Microsoft business subscription
https://learn.microsoft.com/en-us/microsoft-365/commerce/licenses/buy-licenses?view=o365-worldwide&preserve-view=true
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP