rockettree,
IF you are looking for the grey bit of plastic with the yellow spring loaded tab attached, I went through this with Petzl back in 2006. Mine popped off and I discovered a void in the plastic right where a metal tab on the structural part of the buckle is supposed to retain it. This allowed the plastic part to shift enough that the
other side could escape its retaining tab.
I contacted Petzl USA. All I wanted was a replacement grey plastic bit, I would gently prise open one tab, slip in the new one and carefully crimp the tab back down.
Nope, they insisted I ship the harness to somewhere in Illinois or Michigan where they would replace the entire buckle, which would require re-sewing.
So, with a V-Block, bench-vise, brass drift and an 8 ounce hammer, I just put the defective Petzl FAST buckle plastic bit back in and carefully persuaded the retaining tabs on both sides to move about 2 mil toward centre. Never popped off again.
So, if that's your situation, I hope you still have the escaped plastic piece. I was lucky, it popped of in a tree on about the saddles fourth climb, but my eagle eyed groundie picked it out of the debris for me.
If that's not your situation, ignore this entire blowfestration.
Northwind