Like the title says, daughter in law wants to do a wall like this that she saw on pinterest. It looks to be standard chair rails, but what about the arches? Thanks

Thank Google for similar ideas found at .....

Wholesale millwork
Home Depot
Fypon (quality fancy stuff)
Amazon
Crown-molding
American Wood Moldings
Etsy ($60 appears most similar but taller)
MBS Interiors
Ekena Millwork

Those arches are sold at Lowe's, HD, etc. It's not all one piece from top to bottom, you'll build the box out of moulding and instead of using a flat piece for the top you'll use the curved. I think I remember they sell 18", 24", and 36" inside width, often have to order online and have shipped to you/store pickup.

If you zoom in on the picture, you can see the joints

you can get flexible moldings. Go to McCoys or Callaway Lumber. either can get what you need ordered for you.