you need to beat the "Catch a Rathalos" or "Troublesome Pair" quests. the troublesome pair has a MUCH better chance of getting it. that's where i got all mine. if you can't beat that quest, then you have to keep trying to catch him and hope you get one... i think the chances are 1/20 for catching, but troublesome pair has like 19/20 or something like that.