If you have a ps3 controller (you never specified), you're shit out of luck. I've looked EXTENSIVELY for a ps3 -> 360 converter, and frankly they don't exist for a reasonable amount of money. There are two brands that are even worth it (cheap ones are laggy/shitty in general), both cost $60-70 for the converter itself. However, you also need a wired 360 controller to plug in to the converter to make it actually work, which is another 40 bucks. Worthless.
If you have a ps2 controller, get an xtokki360.