One need not actually be a professional to join the PDGA, but one should be interested in playing disc golf and working to improve the game. You can also earn points on the PDGA Pro and Amateur tours, which can qualify you for the World Championships. These terms refer to the angle of the disc relative to the ground, when you throw it. Again, think of a right-handed golfer throwing backhand in this explanation. If you throw it tilted UP, toward you, it will curve from left to right.
Most discs have a natural tendency to fly a particular way, regardless of how they are thrown.
They have a free catalog, and accept Visa, MC, and AmEx. Most disc golf courses have one or two people who hang around there a lot and sell golf discs. There are a number of mail-order places you can get discs, disc bags, clothing, and other stuff from, including Discovering the World, listed above. Everyone has his or her favorite discs, and people often disagree about what discs are good for what shots. My wife loves Stingrays for drivers, but I find they are too understable for most shots.