Here's some Rock Generators I found that may be useful for map makers.
aa_rockGen 1.0 – maxscript
Procedural rock generator for production and real time environments.
Debris Maker 2.0
Free to use procedural geometry generation tool for 3dsMax
Creates Rocks and other stuff.
There is also a site that has a bunch of stuff but you have to subscribe and pay a monthly fee. It looks like it would be useful.
There is some free stuff I found too. You can adjust the downloads to either 8k textures or lower. The rocks come in .fbx to import into Max and they have LODs so you can use lower poly rocks.
I think this could be useful and hope it helps some map makers.