Do you have a local hobby store near by? That would be my first stop to see what type of RTR kits they have and have parts for. If not take a look thru the monster truck section as that will probably be what you’re looking for.

For bodies, most vehicles can take different bodies with a body post change. Within reason of course, you’re not going to fit a 1/12th scale pan car body on a 1/8th scale monster truck. The biggest aftermarket body maker is Proline so start there for bodies https://www.prolineracing.com/pro-line/bodies/