I know this is old, but this is for those, like me, who were looking for the same answer.
My A button was mushy, so I took it apart because the controller was out of warranty. I saw where the little silicon membrane was ripped.
I then contacted PowerA to see if they could sell me the part and they referred me to Amazon or Ebay.
I started looking and found this post, and this is where I gave up looking and started doing: I took an old off-brand PS1 controller apart to see what parts could be used, if any. I ended up finding the silicon mold underneath the D-Pad.
It didn't line up perfectly, but the conductive pads on the PS1 mold fit JUST well enough to where the pads touched the gold electrodes in my PowerA controller. I trimmed the D-Pad piece down with some scissors to be in the same shape as the old piece and then used a piece of cheap double-sided scotch tape to make it stick down in the controller since it was lacking the rubber ‘horns’ that would’ve held it in place through the controller board.
I put the controller back together and it worked! Your mileage may vary, and keep in mind the tactile snap won't be the same.
You can conceivably do the same by buying a newer piece online. It may have a better 'snap' when the button is pressed since my PS1 controller had been used for literal decades.