I strongly advise that you should only purchase captive bred corn snakes from repitable shops and breeders. There are several reasons for this, firstly, captive bred snakes are almost always healthier than their wild collected counterparts, being generally far less likely to have been exposed to such maladies as internal and external parasites and other diseases.
Secondly, captive bred snakes usually adapt more readily to being kept as pets, and finally, the plentiful availability of captive bred snakes reduces the necessity to collect wild specimens for the pet trade, thus relieving the pressure on the natural population. Although corn snakes are not endangered in the wild, it is better to leave them in their natural environment.
Your next choice will be whether to purchase an adult or hatchling snake. Although there are arguments to be made on both sides, it is generally better to start off with a hatchling for several reasons. Firstly, there will be a wider variety to choose from. Secondly, you will know the age of the snake, and if buying directly from a breeder, its genetic background, which will be important if you intend to breed the snake in the future. You will be virtually assured that it has been captive bred, since very few hatchlings are collected from the wild, and finally, you will have the satisfaction of watching your snake grow and mature into its adult coloration.
Whether you are buying a hatchling or an adult, there are several items you should check to attempt to determine the snake's health. Check that the snake appears alert and responsive as you handle them, making sure that they flick their tongue in and out to check out their environment. Check their body weight and muscle tone, the snake should not appear emaciated or have their ribs protruding. The snake should not have any visible scars or kinks in their spine.
Ask if the snake has been feeding regularly and shedding properly. Check the snake's cloaca (vent) to make sure that it appears dry and closes properly. Try to listen to the snake's breathing, if the snake appears to be wheezing, or if mucous is present around the mouth, this may be a sign of respiratory infection. The mouth should close tightly and not display any scars or lesions.
Finally, check the snake for the presence of any external parasites such as ticks or mites. These parasites are difficult to see, but their presence will be indicated by the presence of tiny silvery white specks (parasite faeces) which will be deposited all over the snake's body.