I realize that you may be wanting to 3D-mill foam with a router to eventually move to 3D milling/carving wood. And you haven’t mentioned the foam you want to cut/mill… I personally like the construction foam “Foamular” from HD.
But, for fun, if you loosen your definition of “hot needle” to include “laser” you can get 3D on the non-white Foamular stuff (not white styrofoam) as well…
– David