Like what Zen and the others have said, it is possible to do it yourself, but if you have no experience with this type of modding, and you don't have another controller to spare if you make a mistake, then don't do it. Buying one would be your best option.
Be careful when using one though. Depending on who you play with, they may report you. It takes only a few reports of the same type on your gamertag for you to be banned in Black Ops, and I'm sure Microsoft will be more than willing to take further steps to get rid of cheaters.
In short, its really not worth it. Just play legit.