If you're looking to replace a broken garage door spring, here are some suggestions.
Can you remove the spring and bring it into a home depot or other store to get it replaced? There are many, many different, highly similar kinds of springs and it would be very easy, without the actual object on hand, to get the wrong one.
Of course, once you do get your hands on a spring, there's no certainty that you will be capable of installing it. Garage door repair and garage door installation skills are difficult to acquire and require lots of time, commitment, and bodily strength. There are considerable hazards involved in fiddling with heavy springs under tremendous amounts of pressure and tension.
So maybe you should spring for paying the garage door services professional. It may seem silly to pay slightly more (and you will) for someone else to do something that you feel you may be capable of doing yourself. But when you hite someone else to do it, you are guaranteed that it will be fixed, that no bodily harm will come to you and no irreparable damage will come to your garage door, and everyone is happy. Plus, if you have a warrantee, chances are you won't be paying too much anyway.