Verified answer

I've noticed that the starting set (the one you have when you've just made the DK) and the quest set (the blues you get after completing all the quests) is popular among DKs' transmorg. I've used them on my DK for awhile also.

If you no longer have them, then it's no problem. There's a vendor at the Ebon Hold who sells them.

I'll provide a link below to where his exact locations are.

Dk Transmog

Transmog Sets