As in regulated power supply Les, or just an adaptor for existing supply?
Power supplies range from around $50 at Jaycar/Dick Smith for a 3A job, up to hundreds for more amps and better regulation. You'll probably want to consider if you're ever likely to be powering anything else at 12V and what amps that will take (e.g. dew heaters). I sometimes use a $60 DSE one - they're not really supposed to be used outdoors, so you have to be careful not to get dew and moisture in them or bad for supply and the scope....
I'm pretty sure you could search up threads here on supplies. For example:
http://www.iceinspace.com.au/forum/s...t=power+supply
http://www.iceinspace.com.au/forum/s...t=power+supply
http://www.iceinspace.com.au/forum/s...t=power+supply