This has come up on the forum before and somebody did point up a source for a true white, smooth watercolour paper - I think it's made by Fabriano. It's pricey though as it's a specialist supply and is aimed at people who do things like floral miniatures in watercolour.
As others have said, watercolour paper comes in different ways - typically cold press, "not" and hot press. Hot press is smoothest - it will take even a very detailed stamp, no problem (this is a card made with a detailed stamp from the Saturday Evening Post line and is done on 140lb hot press paper:
watercolour lions).