Daft question, could you not just put two car/lead acid batteries in series to give you the 24v needed? Perhaps two sealed lead batteries in a large deep cycle battery box would work.
There are off the shelf options like this, however they might be a good deal bigger than what you are looking for.
Alternatively any set that runs on 4 x AA batteries could be used by using a step down converter, it would allow you to run the existing sets, though you do note that they are rather dim.
If you are willing to buy direct from China (and believe their marketing), it is possible to get 12v sets (link), though time might be the issue for this year.
Just a few thoughts!