You must be a Registered Number Plate Supplier to purchase Number Plates.

How do I apply to become a Registered Number Plate Supplier?

https://www.gov.uk/register-as-number-plate-supplier/dvla/apply

If you operate from more than one site click

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/824527/v940-1-application-to-register-as-a-number-plate-supplier-supplementary.pdf

To contact the Register of Number Plate Suppliers department within the DVLA, whether it’s about your registration or to update your company details, you can call them on 0300 123 0797 or alternatively, you can email rnps@dvla.gsi.gov.uk