So you want to buy a Raspberry Pi? There a number of official distributors, two of which are RS and Farnell. They sell Raspberry Pis as well as a range of accessories such as power supplies, SD cards, USB hubs, keyboards and mice.
Premier Farnell/Element 14 (Worldwide)
CPC (UK, part of Premier Farnell)
RS Components (Wordwide)
MCM Electronics (US, part of Premier Farnell)
It is possible to buy from eBay and Amazon but they are likely to be more expensive than units from the official distributors.