I don't think that Radio shack still sells that inverter. However, there are a LOT of DC-DC converters on the new and surplus market that may work for your application. Marlin P. Jones
www.mpja.com has a 5V to 12V 1 amp DC-DC convertor for $10 (pn 11666-ps). You would have to contact MPJA to see if it would run on 6+VDC. If not, use a linear regulator LM7805 to drop your 6V to 5V. The other caveat is that your radio has to pull 1 amp or less. This is just one example. Do some searching and you will turn up more options.
