On this page you can get the official Addon Pack which includes all the extra Monster Jam skins and bodies.
You may have to apply to become a member of the group, but usually get approved within 24 hours. As well as a few different tires and tuning settings depending on the truck. After this you can get additional maps (track layouts - stadiums etc.
BeamNG.Drive Monster Jam: CRD Monster Truck Update 1.16 is LIVE NOW. I am a developer for the CRD mod, Mostly doing graphic and textural work for the project.