Said cable is a non-trival matter. There are devices which exist, though they're generally larger than just "a cable", and they are usually somewhat expensive ($100-200 type range). It would probably be cheaper and easier to just get a new video card.
The reason, if you care, is that a VGA 640x480 signal has twice as much data as a NTSC TV signal. To convert, you have to interlace the VGA signal (take every other line, offset each frame), convert from RGB to YPbPr, then QAM modulate the chroma. This takes hardware. A video card with TV out already has this hardware and is produced in mass quantity hence cheaply.
Oh, the device you're looking for is commonly called a "scan converter", if you want to look one up. Be aware that they vary wildly in quality.