Well imagine my surprise! Michael's had sheets of 1/8th inch birch plywood (very similar to the workshop kit quality). I picked up two pieces just to be sure I wouldn't run short, along with a balsa wood square dowel for reinforcing the untabbed front walls beneath the first floor.

I started making marks to cut, but put it away and will probably only work on it again this week if I get tired of tripping over the ever-growing box of dollhouse supplies.