You can buy simple speakers from any audio specialist or Maplin's in the UK (or Radio Shack in the USA)
Only one speaker would mean you would lose the stereo effect. You have to use speakers for right and left to appreciate a good sound.
If your TV or sound system has 'audio out' sockets, buy leads with the correct type plugs. Connect each lead to your speakers and that will do it.
For a proper stereo sound the polarity of the speakers is important, so make sure the connections to both speakers are identical. Also keep the leads to each speaker the same length.