Adobe Photoshop 7 exports to Targa files. But, it is not freeware. Adobe Fireworks is a Macromedia Product that was bought by Adobe. So, its probably why it doesn't allow TGA conversion like the true Adobe products have.
You might want to try Tucows.com for freeware and type in TGA Converter, Targa Converter, TGA, or Targa, or Image Converter into search to see what comes up.
My first search result was this:
http://tucows.com/preview/228797
There seems to be alot of image convertors that export to TGA.