You have to do some homework for that.
Find the main dealer in the city. Or you may call local offices of ZIC or Caltex or Total and ask them who is their main distributor.
Mostly main distributors do not do retail biz. You have to buy in bulk.
Luckily we have Pakistan Oil Store here in Lahore and they do retail biz at same price as in bulk.
Last time I bought 3 Cartons (each carton has 4x4L bottles) for our two vehicles.
Recently bought only 1 Carton. Price of each bottle is Rs. 2130/4L.