One way would be to find where that f2py is that you can import in ipython: After importing f2py in ipython, just type f2py, and it should show you the location of the numpy installation. From that, you should be able to find the f2py binary.
For example, from within ipython, my f2py is in:
/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/numpy/f2py/__init__.pyc
From here, we can see that the executable f2py will be in
/opt/local/Library/Frameworks/Python.framework/Versions/2.7/bin/f2py
As JohnE said, perhaps you have more than one python environment, and the one with f2py in it is not in your path?