Here's what looks like a good source for "shadow box" type displays. Looks like these things (made for--uh--dolls I think) could be adapted to all kinds of dioramas or just a neat way to hang a model on a wall.
formatting link
The link to the boxes is in the lower left-hand corner-- click on "Room Boxes and Turn Tables." (A direct link is too long for some newsreaders, I guess).