So.. my first question would be... why a 3rd party certificate rather than a self-signed certificate?
As for obtaining the 3rd party certificate.... that's kinda beyond our scope here, but I can certainly help you import that certificate once you obtain it.
But I will say this: PAYING for a 3rd party certificate is not worth the expense, IMHO, for the purpose of publishing third-party updates to a WSUS server.
I strongly recommend just making the self-signed certificate and be done with it.