Optimized implementations of MPI may be available for particular interprocessor communication hardware, either from the relevant hardware vendor or from a third party source, and these should be used where available.
Where no such MPI implementation is available, the freely available, portable MPICH2 may be used.
http://www-unix.mcs.anl.gov/mpi/mpich2/
The source code of MPICH2 can be downloaded from this site. It is necessary to configure the compilation of MPICH2 with the directory into which you wish to install the software. There are also many different options depending upon the type of system you wish to use (e.g. options to improve performance on SMP systems). Full information is available in the MPICH documentation available from their web site.