I almost hesitate to recommend this, because you do buy it on Amazon, but given that I've personally used this product with my raspberry pi and had great satisfaction with it, I can't resist. What you really want is the iPazzPort 3D Gyroscope Fly Air Mouse Mini Wireless Handheld Keyboard for Andriod TV&PC. Whew that's a long title, but iPazzPort is short on marketing genius and long on product functionality, fortunately. They have different models available - the more recent ones weirdly apparently integrate SIPphone technology into the keyboard/mouse set, but the model I used was just a keyboard/mouse combo smaller than a candybar that fit in my pocket quite easily. Better yet, it used a standard phone battery, which was replaceable, and the 2.4ghz wireless dongle required no driver installation on any linux or windows system I ever used it on. The mouse is a gyro air mouse kinda like what you'd find in the standard Wii controller, but it proved to be a very functional solution to the idea of a mobile mouse, because the touchpads on these little guys' cousins are just too small to be of much use.
They're not the cheapest you could find, or the prettiest, but I always liked mine and it never let me down. The compatibility was, for me, the killer feature.