Search on the GTS for Vivillon. If you can't find your Monsoon Vivillon there, then try and arrange trades with people from either Japan, India or Hong Kong. I live in Japan, but unfortunately I got meadow Vivillon instead of monsoon. That's all you can do really. Sorry I couldn't be of much help. Good luck.
Also, if you want to know where all the Vivillon patterns originate from, this is the page you're looking for.
Hope I helped!