Tourist information offices
How to find visitor information offices and tourism bureau web sites
The national and local tourism bureau websites are the best places to go for up-to-date travel information.
This is where you'll find the most accurate list of opening times and admission charges for the sights, calendars of events, hotel databases, restaurant lists, information on how to use the local public transportation, sights that happen to be currently closed, and more.
It will also list all the local tourism offices and info kiosks, which should be your very first stop in a new city or town. Nearly all have an outpost at the airport (usually small, poorly stocked, and over-crowded) and in—or very near—the major train station(s).
I find it useful to grab at least a map from the information desk at my point of entry, but then make a point of visiting the main tourist office downtown as well for the widest selection of information and brochures. Also, sometimes the big central office includes a ticket desk for local theaters, tours, and other events.
How to find tourism info websites
Really, as with so much else in modern life, the easiest way to find national tourist bureaus and local info offices is simply to Google "[destination] tourist office" and/or "[destination] tourism information."
You will likely run across a couple of private sites alongside the official or government-run one, but those can be useful, too, so long as you keep in mind that they exist to provide tourism information as part of a commercial or journalistic endeavor. (Then again, this website is also one of those, so perhaps I am partial to the idea.)
There are also a few websites that gather links to tourist offices, though they can sometimes be a bit out of date or subject to a bloody-minded database mentality. Still, useful: