We've posted a prebuilt version of fpm on our package manager for Windows, but please note that our package manager is a paid service.
Using our compiler on Windows, though, bootstrapping fortran-lang.org's package manager is pretty simple. After downloading their fpm-0.5.0.F90 source from https://github.com/fortran-lang/fpm/releases/tag/v0.5.0, you can compile it quickly from the command line:
gfortran -o fpm-bootstrap.exe fpm-0.5.0.F90
After bootstrapping, the build is relatively simple, though you'll need to add a flag to disable some problematic calls:
git clone https://github.com/fortran-lang/fpm.git
..\fpm-boostrap build --c-flag "-D_WIN32_WINNT=0x0600"
I've posted a slightly forked version at https://git.approximatrix.com/cgit.cgi/fpm/about/ that changes the default installation location for tools built with fpm.