These are usually provided as an accessory with 'portable' TVs that
have an 1/8" external Antenna Jack. Stores that sell small 3" to 9"
portable TVs may have them for sale.
Here are two webpages with images of this item.
Portable TV Antenna Adapter.
Use to interface an outdoor antenna or cable
signal to a portable television. 3.5 mm. phone
plug to 300 ohm screw terminals.
(Stock #: TV02) http://www.smcelectronics.com/TVREP.HTM
Down at the middle of the webpage.