You can find this dye, along with other rare dyes in Fairfax Garden. Travel there and look around for Clothing Traders, and when you find them have a look at their items, if you don't come across it the first time, then fast travel to a place far away, and then fast travel back there and keep looking and checking the clothing traders. Also note that the Clothing Trades will not always be there, but be patience, and you will have the dye in no time.