Its available almost everywhere, you can find them very easily at markets like, water pump, Ghareebabad market, Akbar road market , Market Behind Dilpasand (MA Jinnah Road), etc etc.
Buy from the nearest market as the price difference wont be that much. Beware of copies, and when I say beware I dont mean Chinese copies are bad......... they are not bad at all but shopkeepers sometimes sell you Chinese copies at AHL prices.
Genuine AHL carb will set you back atleast 3,500 rupees (not sure of the exact price) whereas good Chinese copy cost a little over Rs.1,000.