Quite simply put, there are thousands of places where you can purchase a pair of binoculars. The one to choose is the one that best suits your needs.
If you are looking for a pair of binoculars for a child, there is nothing wrong with an inexpensive purchase from a local department store or sporting goods supply. Durable porro prisms at small aperture can be obtained for as little as $20 and will start your young stargazer along the path.
If you are a beginner who is looking for a pair of binoculars suited for casual astronomy, there is no reason that commonly purchased ones cannot suit your needs. You will find sporting goods stores and camera shops will usually carry the higher end models—yet there are many companies specializing in astronomical binoculars readily found on-line for around the same price.
But, if you are very serious about using binoculars as your sole source of astronomical equipment, it is best to begin with a dealer that specializes in astronomy-related equipment. Chances are that if you are willing to invest a significant amount of money, you’ll want a model that’s best suited to astronomical applications.
Just remember... Even a very inexpensive set of binoculars can become the most practical piece of equipment you own—be it from an on-line specialist or the corner store!