If you don't find it elsewhere, White Rogers is a major manufacturer of high current DC Solenoids.. including ones for 24 volts. I have 3 of their solenoids in my 4Runner, one for air compressor, another to parallel the dual battery system and another used for my own vehicle anti-theft electrical disable system.

Take a look here.. http://www.white-rodgers.com/wrdhom/common/ptech/relays/relay_06.htm If you decide to go this way, be sure to pull up the specs on the web site and pick one with appropriate current rating... Starter solenoids are sized based on the intermittent duty rating..

You can buy them a number of places on the internet, http://www.grainger.com being one of my favorites....

------------------------
edit....
After re-reading your post, it this is one of the relays that configures two 12 batteries in series for 24v starting, then drops back to the two 12 volt batteries in parallel for charging, the above probably won't help you....