Part 87931-42D50 may work for you: 87931-42D50 Genuine Toyota Part
It has the convex blind spot mirror in the corner. The left and right can often be bought in pairs if you want to change both sides. They are all the rage for 4Runners for they also fit them: Amazon.com: Genuine Toyota 2014 & Newer 4Runner Blind Spot Side View Mirror Glass Conversion 87961-42D20 & 87931-42D50 : Automotive

The mirror part just pops off. Easy to replace.

 

It has the convex blind spot mirror in the corner.

Those are the standard mirrors that came on my '18 LE. I think they are fantastic !!! How the govt could mandate those stupid TPMS sensors, but not something as actually useful as this is beyond me. I'm amazed you don't see these as standard equipment more often.

Edit - the official Toyota Parts website is handy for locating part #'s

 

I believe she was asking about the white piece that the mirror snaps into.

 

The white piece actually is screwed in with a metal washer and a plastic washer. The part that has those wobbly extensions snap onto the motor. Then the washer with the screw go in. Must be aligned correctly for the remote to position the mirror.

Before you try buying a new piece, have you tried reinstalling this piece since it doesn't look damaged from the picture. If you do need to replace it then I would be looking for the entire side view mirror. I know it is more expensive. But, you would be happier if the mirror doesn't fall out if that piece was damaged.

Note: Replacement mirrors don't always snap in easily. I just replaced mine with heated mirrors. The mirror just wouldn't push on. Had to open the clips on the mirror for it to snap on.

Edit: Just looked at picture again. The center of the piece is broken. There will be no way to screw that piece in. You will need a new piece. Still believe a whole new side view mirror would be best. Don't know what other damage the mirror might have absorbed. Good luck.