If you don't mind adding a little weight to the truck then sealed lead acid batteries would be a good choice because they are much less expensive than Lithium or NiCad batteries. They are available in 12 Volt batteries which can be wired in series to create a 24 Volt battery pack. We have these batteries and chargers for them available. Lead acid batteries are available in many different sizes so you might want to look at our batteries page to decide which size would be best for your project. Here is a link to our batteries page: http://www.electricscooterparts.com/batteries.html
A 24 Volt battery pack could be made from any even number of 12V batteries, such as 2, 4, 6, etc. So if a pack of two large batteries is not the right size for the truck, then a pack of four smaller batteries could be used. Here is a link to a page that shows how to wire two or four batteries to produce a 24 Volt battery pack: http://www.electricscooterparts.com/battery-pack-wiring.html
Please let me know if you have any further questions.