there are some shops on m.a. jinnah road just after the tyre shops at the signal. They have 8x4 sheets of acrylic plastic varying thickness like 2mm and 3mm. 2mm is easily bendable but cutting it to proper size is the issue as they crack easily. I havent come across lexan in karachi.
i have used 2mm for rear screen and side panels and 3mm for the roof in my car.