Where is DK sword in inventory

[Enkidoh]

If you're referring to the great sword you were given automatically as part of the quest, it's not a rare weapon so it can easily be disposed or sold on the Market Board, so you may have accidentally disposed of it ages ago.

Not to worry though, as it's easily replaced, you can buy it on the Market Board under 'Dark Knight Arms', plus there is a npc that sells the same weapon (along with weapons for AST and MCH), it's the woman in the winter wear standing with her back to the player near the counter in the Forgotten Knight tavern in Ishgard's Foundation (she is the npc that also provides gear repairs there).

Buying it on the MB might be preferable though, as it is also craftable by BSM and so you can get the HQ version with slightly better stats that way to the normal version gained in the original Job quest (and what is sold at the npc stores).