Xonar DS is probably the cheapest to get DTS connect (if you must have DDL, then read on) DTS connect does the same thing, it puts 5.1 into a SPDIF output for a receiver to decode. If you are using a receiver that has DD, it probably also does DTS, but maybe not hence the need for DDL.
My personal suggestion is to get a xonar DX, its actually pretty fantastic, I've had one for 2 years almost and it beats the tar out of anything I've used before or since. I have also been reading about Creative Recon3d, that looks like its a good card as well, but without the great xonar sound quality.
To your original question - motherboards and their makers must license DDL for each model, so it's not always available on a manufacturer's entire line of boards, if at all. They seem to not include this very often these days.