On the MagPul polymer trigger guard there is a small allen-head set screw that screws into the right side of trigger guard (self threading) that holds it in place (set screw should be screwed into the guard until flush with outer surface of the the lower). In the short term, if you could find out what size and where to get that set screw locally, you could go ahead and fix it (an easy fix). In the long-term, when you do manage to get a hold of PSA CS or the rep here on AR15.com sees your thread, they will ship/mail you the set screw; but it will probably be at least a couple of weeks or more (just guessing based on my past experience with PSA).
If you have more money than time, you could order another Magpul polymer trigger guard from MidwayUSA or Brownell's or other online AR store, and the set screw comes with the trigger guard. Of course, that would take a week or so.
PSA has good products at a good price, but there can be irritations that go along with that... (been there done that)
Just my .02.