in order to finish this collaction and 1 other, i need to find which merchant sells writ : cow

nowhere, the cow doesn't exist in the game yet, I asked for a ticket myself. I was told that and that when it is inserted, it will be in the patch notes.

Nowhere. wait till the announce it at a patch.

you actually get it from sheep xd 

Go to any valencia towns. There you can find it.