Where do I find the only Qwent card in White Orchard region? Who owns it?

There are more than one Gwent cards in White Orchard. But I think you mean the one that you win, which is Zoltan Chivay. You get that by talking to the scholar sitting with two other people in the inn. They will leave, Geralt will get into a conversation with him and when he says he's going now, the scholar will ask Geralt to play Gwent. That is when you win the Zoltan card. There are other cards to buy from the woman innkeeper.

The scholar is no longer in the inn. I tried to find him in town but no luck.

If you missed the scholar before moving on with the game he's gone for good. But don't worry, you can find the card gratis. Go to Velen, to the Hanged Man's Tree where you first started. Go to the tree by the camp fire. There will be a little pouch (loot) on the floor near to the fire. It contains the Zoltan card. SAVE THE GAME BEFORE FAST TRAVELING TO THE TREE. It has been reported that sometimes the card is not there, so reload if it isn't.

Got it by the Hanged Man's Tree. Thank you. How do you know all this stuff? Is there a walkthrough somewhere? I'm still missing 4 cards in Velen and 4 in Novigrad.

Failsafes were introduced in one of the patches (1.05 I think) because too many people were missing the scholar and other missable players like Dijkstra and Lambert, so not being able to complete their decks. Most missable cards can now be found in-game after the fact. I did a quick search and came up with this video that shows all of the potentially missable cards (I didn't watch it all).