I've always been surprised no one at Dolby or DTS has put out a commercial release of demos and trailers. I guess the potential licensing issues have scared them away but there is clear market demand for such an item.
I have a number of demo disks (Dolby, DTS, Auro, etc) either given to me or purchased when I find a good deal. Most are disappointing with perhaps one or two short demos on each disk worth using. I rarely go back to them.
I would recommend creating your own library of scenes from movies or music tracks on disks you own.