They're sold in most larger supermarkets (e.g Tesco and Asda). You can also offer find them in discount stores, such as Poundland.
You need to remember that a USB port provides power (at 5V DC) to whatever is plugged into it. Most USB devices will work via a standard hub but a few might not (because there's insufficient power available via the hub). If you experience problems with a standard ('passive') USB hub, you can buy a hub which comes with its own power supply:
http://compare.ebay.c...mTypes&var=sbar&cbt=y
(For other types, search for 'powered USB hub')
Chris