Most knives (Western ones, at least) aren't specifically left or right handed. Japanese knives often are, and those can be generally broken down into ones with right-or-left-handed
bevels and ones with right-or-left-handed
handles. Western patterned knives with a "handed" bevel will usually work about as well for a lefty or a righty. Handles made for a specific hand can be a little more problematic; in the case of the ubiquitous Shun Classic, a right handed handle won't fit the left very well (although I will say I know many people who think it doesn't matter). A traditional Japanese pattern, like a yanagiba, will definitely have to be made in a left handed version for lefties.
The two best places I can think of off the top of my head for left handed knives are
Korin and
JapaneseChefKnives.com. They have tons of left handed knives (most can be purchased either way).
If you want a Wusthof, Henckels, or Forschner it doesn't make any difference if you're left handed or not. They can be purchased anywhere.