Finding Dragon Strike 20 Mastery Book?

I have enough money to buy it 3 times, hopefully it'll only need one try. Problem with buying it is it's extremely rare in Khaini, only seen it once and it was sold yesterday. I'm on 24 hour 2x drop, just got it about 20 minutes ago, now I'll be on 4x drop during the event. Trying to hunt centipedes.. =/