Buying an inverter is certainly an option. However, if you are reasonably handy it is very easy to convert a standard strand of mini-lights to run on 12v.
First, I would recommend checking out
http://people.howstuffworks.com/christmas-lights2.htm . They give a pretty good overview of how mini lights work.
So, to convert a set to run on 12v instead of 120v (bulbs don't care if it is ac or dc), all you have to do is shorten each series from 50 bulbs per sequence to 5 bulbs per sequence. The easy way to do this would be to buy yourself a string of 50 minilights and a length of zip-cord. Cut your string of mini-lights into 5 bulb length. Lay them on top of the zip cord end to end. Now comes the trick part. You need to either strip the insulation or cut ONE of the two wires at every junction. Splitting a zip cord and stripping the insulation without damaging the wire is tricky but can be done. Splitting a zip cord and cutting one of the two wires then stripping the ends off both sides is easier. Connect one end of both segments to the exposed wire(s). The magic, and potentially tricky part, is you MUST alternate wires at the end of each segment.
Using crummy line art, what you will have is
[pre]
+ ---v-----------v----------
|.....| |.....| |.....|
- ----------^-----------^---
[/pre]
Which, if this comes out, shows you have 5 bulbs (in the middle) from + to -, then 5 bulbs from - to +.
Anyway, you should get the idea. It is a bit of wiring but it saves you from having to an inefficient inverter and potentially lethal voltages.
Of course, you could just buy a string of 12 volt rope light from
http://www.christmaslightsetc.com/categorydetail.asp?CategoryID=131 /forums/images/graemlins/smile.gif
