Cheapest i could find delivered for this PS3 Official Wireless Keypad -
Attaches to the top of your PS3 controller for ease and comfort. Push the 'cursor mode key' and, the surface of keypad can also be used as a touchpad just like a mouse! Also features shortcut buttons to Message Inbox and Text Chat. PS3 controller not included.
Also (Which i forgot to do myself: Quidco!)
Edit: Also just found at play.com for same price - http://www.play.com/Games/PlayStation3/4-/8115006/Official-Sony-PS3-Wireless-Keypad/Product.html
(But less cashback on quidco)