All the "big box" stores sell kits for toilet repairs. Get a good idea of what kind of guts are in your toilet, and then go buy a kit that matches as close as you can.
If it is purely the flapper and the flapper alone that is giving you problems, they sell replacement parts for that as well.
NOTE: Chlorine is really really hard on rubber. If you are using chlorine tablets in your toilet tanks so you won't have to clean them as often, (you should still clean them, cause the seat and stuff get gross with or without the tablets) you are also destroying the guts to your toilet tank.
Just clean the toilet on a weekly basis and save the plumbing parts from early retirement.