A Joypad Enhanced Kit for PS3/360 Controller it comes with:
Joystick extensions - these increase your range of movement making aiming easier.
D-pad extensions - makes using the d-pad easier .
Trigger extensions - makes hitting the triggers easier.
HMV sells the trigger extensions for £2.50 alone and the joystick extensions are sold for around £5.
FocalPrice are really good in terms of price but expect up to 3 weeks for delivery.