For 12v power, I don't know of a better solution than lead, unfortunately. But maybe others do...
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2052I use one of these to power my pre and a/d, and it's a hell of a lot nicer than lead. I actually use the 5000 mAh model, but I can't seem to find it on their site anymore.
You could probably get away with one of the 4500 mAh units, but I'm not sure how much the Tascam draws or how picky you are about extra juice. FWIW, I can run my 316 and my Mini-Me for a little over 5 hours on the one I've got.
You will have to put together your own cable with one of those Tamiya connectors, but that's a piece of cake.
If you've got a box that only likes 12V (like I do), I think this is a really great option.