I very recently learned of another option -- a custom sized bezel. So you find and buy a microwave oven that's smaller than your opening, and then have these guys make you a custom-sized bezel to fill out the space. The bezels are made of aluminum and are available in black, white and stainless steel finish.
The company is located in Irvine, CA and this is all they do -- make custom oven bezels.
The company is Microtrim.
Microwave Trim Kits - Micro-Trim, Inc. I haven't bought from them, but photos of one of their custom bezels in a Chinook look great. That guy said it cost him around $80 about 10 years ago. So I'm sure they cost more now. But it's a way to get the microwave oven you want -- and have it look professionally installed.