From an old DOS computer. Besides that you will need a VGA card, a scan converter (from VGA to NTSC) and two downloads.
Download showjpg from here:
http://www.pictview.com/showjpg.htm
And download PM5544.jpg from wikipedia.
Set up your autoexec.bat so that it will go into the program upon bootup. Then disable your keyboard in BIOS and disconnect it.
Most test generators these days are software driven rather than hardwired circuitry.
Scan converters can be either internal or external. Some VGA cards have an NTSC output.
The Chinese have a scan converter on Ebay for ~$10 (VGA2AV).