T, the answer to your question is a bit of both. Many places order chicks for much of their season, then buy mature birds if more are needed throughout the year. If you see the prices many of these places get it can seem pretty appealing. The issue is it's pretty hard to make this work successfully as a hobbyist. The guys who are doing this professionally raise thousands of birds and can get them to the farms when they are needed. They also have the means to process birds themselves in a down year. For instance, when winters have a lot of snow, most of the cover even at "gamebird resorts" is knocked down. This significantly shortens the season for outfitters. Thus, they need fewer birds and pheasant producers are often left with a surplus. If the winter is mild, such as what we've seen so far in much of the US this year, the season is extended and producers often need to have pheasants ready to meet the demand.
Outfitters contract with bird producers that can get them what they need when they need it regardless of conditions. It's pretty tough for someone to get started in this business and be successful without significant resources. Sure, you could raise a few hundred or maybe a thousand birds on your back 40 acres. If that is something that interests you, perhaps get in touch with local dog training groups. They always need birds and maybe that would be a niche you could fill. "Gamebird resorts" are another beast. Many go through thousands of birds a year and it takes a lot of equipment to handle that. For more info, check out this site - http://www.pheasant.com/
Take a look at their facility. It is incredible! Good luck in your endeavor!