Stephensonstraat 18, near the Amstelstation, phone nr. Sure they would rather sell you something than answer your questions, but the VVV Tourist Info is a great information source that dwarfs other countries offerings. The old one is still in front of the station across the tram tracks. Traditional Dutch service is somewhat inattentive. Good service deserves a tip.
Bad service deserves nothing.
It's best to give the tip directly to the server or bartender, instead of leaving it on the table or bar. The posted price or 50 cents. Leave something every time you go and feel free to ask for change if there are no 25 cent coins on her plate. They line up at taxi stands and wait for you to come to them. One of Amsterdam's shopping highlights though are markets.