Stamp #099 can be obtained when acquiring "first contact". This stamp is easy if you know what to do. Wait till midnight and head past the large football to the north-west plains of Sandpolis. You will see some lights; follow it and you'll see a UFO, but catch up as it goes quick. Go into the tractor beam and you'll be automatically inside and you'll encounter a silver Audi TT who will speak to you in an alien language. Pick the "Yup" option and the alien thanks you for being contact and he will give you a UFO pattern for your garage and when you get outside, you'll get a stamp.