True, but you can get DTS encoded CDs - what you are looking for is DTS encoded WAV files.
They do exist - but, and how's this for helpful (sorry), I don't know any sites you can download them from.
In any case, the ones I know about are just test tones/sweeps etc, but they would be fine for a soundcheck (that's what they were created for).
What you really want is somewhere that has MP3'd them.
They wouldn't be playable by a normal MP3 player, but the compression would make them reasonably sized for download (I've actually no real idea whether you can actually compress them this way though - supposition on my part) - you need to get them, convert them to WAV files (MP32WAV), and then burn the resultant WAV(s) to a CDR.
Then you'd need to play them on your DVD player (which would have to be CDR compatible - not all are).