Replacing windows is an easy home project. Most any style and size can be bought out of shelves and special ones can be custom-ordered for little additional cost.
When shopping for replacement, make sure to note the size of the openings where the windows will be installed as well as the dimensions with the window frame in place. Even with similarly-sized openings, measure individually as minimal differences can cause problems when installing.
For small home jobs, like a single broken window to be replaced, it makes sense to just visit the nearest hardware store with your exact specifications in tow and buy the item from them. They should be easy enough to install without hitch. For bigger jobs, like installing bow windows in an outdoor patio, you might consider checking with a professional contractor.
In such cases, you can call up contractors right out of the Yellow Pages or seek them out online. There are a few sites that offer free estimates from different contractors based on specifications you give them. Window Buyer does this in an automated manner - with free instructions on questions to ask prospective companies during follow-ups as well as a contractor evaluation guide.