You can get it in the bandit den. If you know how to get there , just go there and buy it from the npc. If you dont know how.. just go to Jangan. Near the stable ( Your looking at the stable, then go behind the stable straight till you see a woman npc smokin

Talk to her and she will teleport you to Bandit Den. There just go to the npc that sells the teleport scrolls and the suits. Buy the one you need or like more. There 1 str or int adding that comes with the suit.
And Btw. Does any one got alredy lvl 7 thief or at least 6? Can you post a picture of your thief lvl and can you buy a newer suit ( Maybe the higher lvl thief`s can get it, im just 3rd -.- )