you can buy the parts at
http://www.ifixit.com/ or
http://cnn.cn/shop/, but beware taking apart your phone takes a lot of patience
i have orderred from both of those sites, cnn seems cheaper (but based out of hong kong, so shipping is more), and ifixit carries tools that make it easier to take apart your phone
i personally would try to decide if the scratches are worth spending a couple hundred dollars.. apple will probably fix it for around $250 and i imagine you could do it yourself for about 150
oh and also if you do decide to take it in to apple, a restore would be a good idea (i am assuming that your phone is jailbroken), and be sure to backup your phone first