Dwight mentioned acrylics that are impact modified; another material that comes to mind is crystal styrene, which has excellent clarity and is reasonably impact resistant. I have molded display cases for models out of crystal styrene and have had good results. Most display cases don't get tossed around too much, so I would think that whatever material you choose, it should be the most cost effective. I would definitely not recommend polycarbonate, as it is quite expensive.
" Imagination is more important than knowledge." Albert Einstein