I suggest taking your Tape Measure along with an exact note as to the size to Microwaves R US (Someplace like Best Buys or a competitor.. Target might work too) And start taking measurements.
When I had to replace my original "Dometic" (Made by Lousy Goods that is LG) after it failed dangerous (A feature of LG products is failure dangerous) I went to Target and picked a Oster up.. not my first choice.. In my case the size is "negotationable" But the wild thing is when I got the Dometic out and the oster on the counter (Fit nicely) I put the Dometic in the box the new one came in ... in the packing material.. it fit perfectly... Amazing (the new one is 0.1CF larger inside by the way)
The moral is.. Take your tape measure to the store and start measuring display models.