that kind of question will get u ignored around here. but i'm nice so don't worry. On a stock car they dont add much at all, but sound nice. Get them at any exhaust shop. prob cost u like 130 installed? i think thats what i paid. do it for the sound, but don't expect to feel much hp gains from them.
i had a shop intall my o/r h-pipe i bought online, with there flows and new tips, all cost me $200 plus tax. considering i went from clogged cats with stock mufflers to my current set up, i felt a big difference, not sure how much, but it was noticable.
I bought mine from Baxters then went to an exhaust shop and had them installed. I still have them today and they still sound great I have them setup now with mac headers, mac off road h pipe and the flows dumped befor for the axle. This setup is loud