Cooking sherry is low-quality sherry with salt added. There is no reason to buy cooking sherry. Just purchase any inexpensive sherry and this will give you better control over the sodium content of your food, not to mention, a better flavor.
The alcohol content may vary from brand to brand but typically it's between 14% - 17%. In comparison a white wine is 12% to 13% and red wines average about 13% +. Regular drinking sherry is also typically 17% alcohol.
You can buy cooking sherry at most grocery stores. It is not considered an alcohol product so you can purchase it if you are underage. Real sherry wine can only be purchased by an adult.