Thanks @Empire_Phoenix, by "can't make the levels myself" I mean that I have no 3D modelling skills whatsoever (currently, although I'm slowly making my way through Blender tutorials) and am completely leaning on the pre-existing + open source models/assets so that I can play with jME3 and learn the framework.
My characters/models need a world to run around inside of, hence this question.
There was a Blender Level Editor add-on that was advertised on YouTube but the download link was never posted and so it was either abandoned or hasn't been released yet. I downloaded & installed GtkRadiant (hoping I could export a Quake III model into Blender, then convert it into a Blend file and use it in jME3) but the app is completely buggy and unuseable on a modern Mac.
It doesn't look like there are any other free level editors/world/map-builders out there (that are Blender and/or jME3 compatible), and so I'm resorting to using any pre-existing maps that others have built and that I could use for hobby purposes.