This is really the simple part. You can't take a step sideways without being near a reptile show. These are great b/c allows direct inspection of the animal. Many people do well with internet purchases. Can't think of last time I heard this go bad in terms of shipping. Many reputable breeders and distributers on internew-- most of whom are listed on this site.
Next issue is choosing the one that is right for you. It's common the read a message where someone bought a tort at a show or pet store....and then want to know how to take care of it. Researching the right tort for you and your circumstance should be given great effort. Again, internet sites that discuss specific care needs are plenty and invaluable. I spent several months for deciding on a breed and husbandry needs. Individuals that participate in this online forum have ungodly amounts of knowledge about species and subspecies. Once you get the basics down, others can answer your more specific questions.
Good luck on your journey.