Other Sources of 3D Models
You might feel that you are not up to the task of making your own 3D models, which is totally fine. There are a few other ways to approach this.
Solidworks, Sketchup, Maya, 3DSMax, Cinema4D, Rhino, Houdini
Blender isn’t the only way to create 3D models. If you already have experience with other software, go ahead and use that! I personally use Blender because it is free, fast, and very powerful. Most 3D software is prohibitively expensive if you aren’t a either a student or employee at a major company.
SketchFab, Free3D, CGTrader, Turbosquid
There are a number of websites where artists make 3D assets available for download. Most of these will not be too useful, because you are hopefully looking for meshes and materials specific to your map idea. Furthermore, most cost real amounts of money.
Unreal Marketplace
Epic Games, the parent company of Psyonix and creator of Unreal Engine, integrated a marketplace into Unreal Engine 4 for easy content sharing between game developers. There is an impressive amount of content available there both for free and for money. For example, Epic made public all of the assets for its game Paragon However, these assets are .UASSET files, meaning that you will have to first place them into an Unreal Engine 4 project and then export them as .FBX files
Additionally, the exported FBX files might not be compatible with the older engine. Autodesk has an old tool (almost as old as UDK) that should take care of that.
Again, this might help in very specific ways, but this won’t work for everything.
Contract Work
The standard way to get stuff done when you don’t want to handle it yourself is to find someone else who can do it. There are a number of websites such as Fiverr, Freelancer, or Upwork for hiring someone, but unless you have a very clear vision and have budgeted for your project, these options are not for you.
Collaboration
The mapmaking community has a number of capable 3D artists who are there to help, either with advice or assets. However, it is unfair to expect that anyone do work for free, so please avoid requesting that. If someone offers to do work for free, then be grateful for that.
I would love to see more collaboration happen in the mapmaking community, because not everyone wants to learn the entirety of Blender just to make a simple custom map. I had the pleasure of working with Panic on Air Race, and thanks to gidek's amazing Kismet work it has really taken on a life of its own.
Discord Server
If you have an idea and are looking for collaboration, check out the #collaboration channel in the Discord server and someone might be able to help you. If you have an idea that's way too big to do on your own, you can always post it in the #projects or #map-ideas channels.
Rocket League Itself
But the ace up our sleeves? Dummy Assets.