Replacing extension springs is a little different than replacing torsion springs.

There is a little less danger involved but still can be very dangerous if not treated with respect. I do know that retail hardware stores like Lowes and Home Depot will usually carry these types of extension springs but not in all regions of the country since you will not find any one piece garage doors in regions where they get a lot of snow. Since one piece garage doors flip out when they open it would be very difficult to open a one piece garage door when there is a couple of feet of snow on the ground. The places where you are most likely to find these springs in a retail hardware store is probably closer to the west coast. The most common extension springs for a one piece door are made by Holmes Co. They have an orange tag on them and the spring size is stamped on this tag. The most common size is 728C. The 7 is the coil wire size and the 28 is the length of the spring. Most of the time an average size wood one piece 2 car garage door will have 4 of these springs. Two on each side.

change these springs- on your own?

If you are going to attempt to change these springs yourself then there is a few things you need to know. Door has to be all the way open and supported with a 2×4 to hold open door while you make spring change. If a hinge has come apart from the wall than do not go near it or try to operate the door. This can cause serious injury.  Also there is a lot of good information on YouTube about replacing garage door springs where there is a detour shin or extension springs if you are going to attend this yourself and have never done it before then getting a little bit of education on the subject might be a good idea since it is kind of a dangerous thing to do.

match up the new springs

You will have to match up the new springs with the old springs exactly or else you are going to have issues with the opening and closing of your garage door so if you think this is too much for you then do not hesitate to call a garage door company to replace your extension springs since it is such an easy and inexpensive repair for a one piece garage door it might just be easier to hire someone then attempt to do it yourself. Is not usually too expensive to change these types of extension springs on a one piece door so you might want to ask how much it would cost to replace all of your springs since you have one broken one you may have another broken spring pretty soon so it is just something to think about. Just always be careful and treat Springs with respect and for good measure try to get a friend or family member to help you with this project