Google or cabelas/Bass pro shops has them.
This will not supply you with enough charge during the use (i do not believe). if you want it for charging while not in use it will be decent.
If looking for more longevity try and wire two 12 volt battery in line. Not 24 volt but you can wire in line to get 2/12 volts battery's on one motor. See online (helpful). Size of your battery's is important too.