You have a couple of perfectly good answers here from DJBunk and Mr.Wizard. However, it appears from your question that you need a better understanding of why or how you would buy a rope with certain specs in terms of elongation. The elasticity of a dynamic rope is a compromise. If it's too elastic, then when you take a fall with a high fall factor, you could end up hitting the ground. If it's not elastic enough, then it will stop a fall so abruptly that you'll be get hurt by the force of your harness on your hips.
For example, say you're just starting the second pitch of a multi-pitch climb, and your belayer's station is an anchor on the side of a vertical cliff. You climb some distance above your belayer, and then just as you're about to clip into your first protection, you slip and fall. In this situation, your fall factor is 2 (the highest possible), there is nothing for you to hit, and you're going to fall past your belayer. If you had a choice in this situation, you would choose the most elastic possible rope, since it would decelerate you as gently as possible.
On the other hand, suppose you start climbing from ground level, clip into your first protection at the first opportunity, 3 meters off the ground, and then after another 2 meters of climbing you're getting ready to clip into your second opportunity for protection. It was unwise to get yourself into this situation, because if you fall while trying to make this second clip, the rope will only come up taut while you're 1 meter above the ground, and it will probably not stop you before you hit the ground. In this situation, you're wishing your rope was a little less elastic.
In reality, you only bring one rope for a day of climbing, and on any given day there may be some situations where a higher elasticity would have been better and others where you might have preferred a stiffer rope. You use a dynamic rope that's designed for this general type of lead climbing, and you carry out your climb in a style that is basically a matter of habit and tradition. These habits and traditions are based on the typical characteristics of a standard dynamic rope.
Suppose hypothetically that dynamic ropes weren't standardized, and you bought one that was more or less elastic than the typical rope. This would be bad and potentially dangerous for your climbing partner, who has built up habits and expectations based on experience with rope that has the standard specs. If you're talking about lead climbing in the gym, then the bolts on the wall are set up at heights so that with a standard rope, you're likely to be OK if you fall while trying to reach the second clip.
I don't really know of a lot of reasons to own rope that is more or less elastic than a normal dynamic climbing rope. The only example I know of is that if you're really into canyoneering, you might want to bring a static rope for a day of canyoneering, because you don't want a bouncy rappel, and the rope is never going to be loaded with more than your static body weight.
