You're guaranteed to get two when you visit the desert valley after it's been blocked off (post questboard unlock). Go up to the Northern Settlement, then go north-east towards the gate with the few workers mingling around.
Thanks so much. I got them. Another question, after I try to synthsize "Dark Latex tatters", wizard says "you have no need for this item". Is it because that I have already have "Skimpy Cover" armor set?
That's correct, the 'dark latex tatters' is another way of getting the set if you've missed out on the bandit ones! If you already have the garments however, you won't be able to get them again.
