Walaikumaslam student123,
If you have a lot of possesions in pakistan it might be worthwhile to look into shipping it over ocean freight. You may be suprised at how cheap it actually is to ship things. Ocean freight is calculated by the cubic meter, not by weight like airlines. Unlike the airlines you are looking at 20 - 40 days for your shipmant to arrive, so a little planning may be required to get your posessions from the port to your final destination. If you have over $10,000 worth of items it may be worthwhile.
Also in Karachi you can get boxes from Bolden Market.
ASyed