As Colin suggested, if you do not go through an authorised seller, they will be illegal.
The sale is documented at the garage and with your driving license and a copy of your V5.
The plate must also display the postcode and garage name of the supplier
You could get plates on ebay, but they will in all likely be illegal. Like everything else, the plates will be logged and available to the cops at the roadside. Just like insurance and mot data. Even when and where the plates are made up.
Everything is online and available to law enforcement.