As Paul said, go to
vbulletin.com and go to their
ordering page. Once there, choose your currency (GPB, Euros, or USD), select as many licenses as needed (most likely one) and whether you want an owned or a leased one (I'd suggest owned). Select if you want phone support, branding free, or installation services. Then fill out all required info as specified by subsequent forms. For more info, see
here.