You can plug an extension speaker into headphone socket, but you may need to control with TV volume control. Usually wire up a Stereo pair to a stereo minijack ... 2.5mm or 3.5mm, or if it's older TV a mono jack. Split jack to two speakers connected in parallel.
Should work ok.
Use some 2 way car speakers or small 2 way hi fi speakers.
As Above ^, but if a headphone socket is the only option then any computer speakers would also be a good choice.
If you have a fairly new flat screen TV there may be other options though, so, posting the make and model number would help to figure out the options available.