HobbyTronics.co.uk add a Raspberry Pi section
If you're in the UK and eager to get your hands on Raspberry Pi accessories from a good local reseller then you might want to check out the HobbyTronics.co.uk Raspberry Pi section.
With GPIO ribbon cables available in two lengths, plus the Humble Pi Prototyping Board, the Slice of Pi prototyping board, the Serial VGA Monitor Adaptor and a glut of handy compatible ICs for expanding or protecting your GPIO port, it's a pretty good place to pimp your Pi.
I also recommend picking up an Arduino breadboard kit, plus a handful of any components which interest you. I've had a lot of fun hooking the Arduino up to the Pi, and they make a great pair with the strengths of one complimenting the weaknesses of the other. I run my own "breadboarduino" at 3.3v so it can interface directly with the Raspberry Pi