It would probably not surprise you to learn that over 80% of sportscasters are male, but it's interesting to note that female sportscasters are narrowing that gap every year. In some events, female broadcasters are well represented; ice skating and gymnastics, which receive more coverage than in years past, are two examples of this. Other events, such as football, rugby, basketball and hockey, have stayed more true to the stereotypical male broadcaster, though newer productions such as those broadcasting on the internet or via podcast, do not hold as tightly to this business model. The very largest sporting events, such as the Olympic games, can literally keep thousands of sportscasters employed for the duration of the competitions, if you consider each sporting event and ceremony will be broadcast in dozens of countries, translated, and then disseminated to local broadcasters, internet and podcasts, and other mediums.
Television networks are one of the largest employers for full-time sportscasters. Major television networks, as well as sport-specific channels and newer "niche" television channels that specialize in only one sport, keep a number of sportscasters and commentators on staff, typically on contract. Radio work is still common though not as prevalent as it was in the past. Webcasting and other internet-based sportscasting opportunities are becoming more widespread, however currently these positions tend to be not as lucrative as other mediums.
The typical sportscaster's "office" might be the stadium press box, a television studio, right on the sidelines or even in the locker room, interviewing players and coaches after a game.