In my last two homes they were built to fit the space. A carpenter could do this.

We used the Rejuvenation cabinet and carved the extra 1” from the adjacent blocking since it’s 19”x24”—people here gave me instructions on how to do it years ago so that thread might still be around. It was a relatively simple DIY project. We used the same cabinet in a new bathroom recently since I really like it. They’ve since moved the production overseas but I think the dimensions are the same.

"They previously had recessed medicine cabinets with space for a 24" x 18" cutout."

Come on, man. 24" wide or 24" tall???

101 DIY project & get the exact size you need. I made my own. 5 of them between the last two homes. Out of scrap hardwood & Plexiglass from a huge early-gen computer that I rescued from a dumpster. Basic box with a plant on back. Drill holes for shelf supports before assembling the box. Cut Plexi shelves. Add a door. The first ones were doors made of a piece of 5/8" plywood with 1/4" solid wood edge banding, mirror held with clips and hung with shutter hinges. Later upgraded the doors to hardwood mirror frames.

@wdccruise what do you think? how many medicine cabinets have you seen that are wider than they are tall? Let alone, how many do you think they were making in the 1920s that have more width than length. 24" tall....

@ci_lantro Sounds like a little bit more work than I would like to do, but maybe a last resort.

@girlnamedgalez8a I am the carpenter in this situation. I didn't realize how non-standard these dimensions were.

@artemis78 do you have link for the product you're describing?

I had mine made by the guy who did my cabinets. It has a door that matches the vanity door in miniature. I didn't need it to be mirrored as the original one was since I had plenty of mirrors after the remodel.

Not sure if Houzz will let me post a link but I’ll try: https://www.rejuvenation.com/products/mendenhall-medicine-cabinet/

Or search for Rejuvenation medicine cabinet—they make several variations, and all but one are the same dimensions. (Wright is the one that’s different, I think.) We have two (one purchased for a new bathroom last year and the other for our existing bath 9+ years ago) and really like them; initially got one from another store and returned it because the quality was disappointing. We were DIYing the first installation and building it from scratch was well beyond our skillsets, but this was pretty simple. We are in a very high cost labor area so would have paid considerably more to have a carpenter build one. YMMV if you’re in a lower cost area.

Mine were built between the studs in the wall. The only thing on the sheetrock was the frame & the door.