Add Sata drive to HP desktop machine

I have one of these:-

http://www.pcworld.co.uk/gbuk/hp-pavilion-p6-2200ea-desktop-pc-14134708-pdt.html

to which I have added a Sata drive in a USB caddy.

I now want to buy another p6-2200ea but want to add a Sata drive internally onto a Sata connecter on the main board.

I think I need an internal adapter/caddy. Does anyone know what part I need and where I can obtain it in the UK?