That’s a really bizarre peice for someone to steal.
I don’t have the part number but I have had good success in the past looking up P/N’s on part websites, such as oemacuraparts.com. You could probably start there.
It's called a 'hinge cover' and is a part that gets removed, cut, and reinstalled if a roof rack is to be installed. I suspect yours likely just flew off and is laying near a road somewhere.
I detailed the removal and install of this part in a thread I started here a long time ago for a 'roof rack self install' and it includes some pics. It might be helpful for you. It's very easy to install a new one yourself.
I just installed an OEM roof rack on my 2014 MDX. I purchased the roof rails and crossbars through an Acura dealer online at quite a discount from retail. It was about $350 for the rails and $175 for the crossbars. You can find it easily through Amazon by searching for "2014 MDX roof rack"...
Just had to buy this a few months ago. It comes off sometimes if you go through a touch carwash. Mine came off because it was already loose from the white paint defect causing a roof repaint and the body shop didn't know how to put it back on.
Ours came off in a no touch carwash AND we did have the white paint defect BOTH on the top of the car and the driver side passenger door. Well, until I read this I did not know the part name or where to buy. Thank you!