I bought a new microwave for my '97 SMB, and also had to search around to find one that would fit. I ended up buying one at Wal-Mart, and it fit perfectly. I don't have it to look at right now, but can get the info from it and post back later.
Mine had (from SMB) a length of small aluminum bar stock at the front and rear of the locker to hold the legs in place. I re-used it but had to leave the front bar off, put the microwave in angling it upward to get it in past the "entryway" (which is smaller than the actual space), then drop it down with the back legs *behind* the rear strip. Only then could I re-install the front strip, then slide the microwave forward (lifting the back legs over the back strip), and into place. Little SMB puzzle
BTW when I say "forward" I mean toward the front of the compartment (whichever way it faces in your van). May not be forward in relation to the way the van travels down the road.