An interesting approach might be to use DC -DC converters from the electronic industry. For example, this one avaliable at Digikey.comhttp://rocky.digikey.com/WebLib/Artesyn%20Technologies/Web%20Data/BXB150%20Singles.pdf
These are capable of outputting 12 volts at 12 amps or so. They are somewhat pricey, $145 or so.
Another option is an unregulated converter from newenglandsolar.com - they have a few variations on page 56 and 57http://www.newenglandsolar.com/catalog_pages/catalog57.htm
although again, somewhat pricey.
Some ebay possibilities- there are often very nice converters abaliable for a song, but you have to kinda know what you are doing to use them.
As an example, searching dc converter in the business/industial section, this guy has some that convert 24 to 12 at one measely amp, but for only 12 bucks. You could distribute them around using more than one...
Item number: 7618525325
My favorite way of dealing with this is to use two batteries hooked in series for your 24 volts, and center tap them to get all the 12 volts you want. The trick is to use two separate 12 volt alternators to charge the two batteries, one hooked from ground to the center tap, and the other one totally isolated from chassis with nylon mounts, it's frame hooked to the 12 volt tap and it's output hooked to the 24 volt end of the batteries .
Doing it this way, each battery has it's own alternator and you can draw whatever you want to from 12 or 24 volts without danger of unbalancing the batteries, as the alternators being dedicated to each battery keeps them happy. I've done this with 24 volt and 36 volt systems and the results are great. Plus a 2nd alternator might be a bit to install, but it's a LOT cheaper than a 24-to-12 converter and you can have virtually all the 12 volt current you want instead of being limited to 10 amps or so.