You can keep things alive, like the radio, by connecting a 12v DC supply through the cigar lighter socket - then remove the battery.
You can make your own "memory keeper" with some wire, battery holders, batteries and an extension lead that fits the lighter socket. You need to be handy with a soldering iron and take care to get the polarity correct.
In the UK Maplin will sell all the bits you need. Eight C batteries in two holders in series should do it.
Commercial memory keeper devices are also available but very difficult to find. Sealy / Draper £25 / £13
Bucko, October 2008