Dear Christopher,
Thank you for posting this question in Microsoft Community.
I understand your concern and to my knowledge, if you want to buy Office Professional Plus version, you need to find a Microsoft Volume Licensing parnter to get pricing and purchasing details
you could access the Microsoft How to Buy guide to find a Microsoft Partner in your region:How to Buy | Microsoft Volume Licensing
If you need any further assistance or any misunderstanding from our end, please feel free to post back to us.
Thanks for your patience and understanding. Have a good day!!
Best Regards,