How do you get a surfboard in soul silver?
You don't, because there is no surfboard item. You surf on water by using a Pokemon that knows Surf.
The Silver Surfer got his board from Galactus, a fearsome cosmic entity that destroys planets. The surfboard is an extension of Galactus's power. It was given to the Silver Surfer in an agreement to serve under Galactus as long as the surfer's own planet remained unharmed. For more info, go to: http://en.wikipedia.org/wiki/Silver_surfer