Just get the source, and compile Boost yourself; it has become very easy. Here is an example for the current version of Boost on the current macOS as of this writing:
Unpack and go into the directory:
tar -xzf boost_1_50_0.tar.gz
cd boost_1_50_0Configure (and build bjam):
./bootstrap.sh --prefix=/some/dir/you/would/like/to/prefixBuild:
./b2Install:
./b2 installDepending on the prefix you choose in Step 3, you might need to sudo Step 5, if the script tries copy files to a protected location.