Vector network analyzer for SMPS feedback loop measurements
Maxim has a good app note on winding your own isolation transformer for this application: Maxim app note

It's not very easy to make these kinds of measurements with a signal generator and scope.  The network analyzers have narrow moving digital filters that allow comparison of the gain and phase of the output signal vs. the input signal in the presence of noise and harmonics.  

I have access to these types of analyzers at work, but for my home lab I was able to find an old HP 3562A on Ebay for $400.00.  It works fine for loops with crossover up to 100kHz.

You might also try an old gain/phase meter like the HP 3575A, which you might find even cheaper, but I'm not sure how good the results would be.

If you want to get ambitious you could get a signal acquisition board for a PC and write a bunch of code to do it yourself.  Would be a fun but tough project.