I don't think there's a way. Sorry.
You could, of course, make a channel in Photoshop for the transparent layer, and save it as a second targa file. But you'd need to use the second file to mask the first in some other app (such as Photoshop) to get the transparency back again. Might as well just save a Photoshop file.
You can have an alpha channel for targa files in Photoshop. They wont show up in Preview though like GIF or PNG. They're an additioanl alpha channel that shows up in your texture.
Your file must be in .tga first. Open it in PS. Channels panel - Add new channel. It'll get added as 'Alpha 1'. If you fill it Black, you're hiding content, adding alpha. if it's white, you're negating alpha, showing content - pretty similar to layer masks.
When you save your .tga file again, ensure to check 'Alpha Channels' in your .tga filetype save selection.
There is a much easier and quicker way round this issue if you have After Effects in you Adobe suite.
Save out of Photo shop as a psd as normal, open psd in After effects and save out as a Targa image sequence with alpha, hit the render button and you will get what you are looking for. However, you do need After Effects.
Wow this is an old thread. Old enough I have to rethink my post. Adobe did some changing with the tga format, so it doesn't behave like it did before. Frankly there is not much of a use for this format anymore. It was use dramatically in the video field. but with psd, tiff and png there is not much of a need anymore. But if someone is using older software, it maybe required, but the transparency may not behave like it did and therefore could cause issues.