I am assuming you need the sprocket for the rear tire. Best bet is to take the wheel or sprocket into a bike shop to make sure you get the right size (yes they are readily available). The problem is, provided you have the right tools, it will take some time to change it over (1-2 hours). Without the right tools, it could take much longer (if you can get the old one off at all). Depending on the problem you might be able to rebuild the old one. If your missing a tooth it will have to be replaced, but if it's a bearing problem, you can dismantle it and repack the bearings with some fresh grease and be good to go (I've done it a few times in the past with good results).