The best option, if possible, is to trade them in for the 12 volt version.
Transformers only work on alternating systems, not on aircraft DC systems.
However, there are DC to DC voltage converters available. Here is an example on ebay:
http://tinyurl.com/12-24-Converter
I am not necessarily recommending it. Be aware that sometimes companies from China exaggerate the capabilities of their products.
Joe