Gardening. (Dhaka)

Hi Everyone.
I am both new to Dhaka and gardening, big challenge hey?! Does anyone have any advice on where to buy seeds and soil from?
I am mostly looking at lettuce and parsley, other herbs and tomatoes. Stuff that is going to be peeled and cooked I am not so worried about growing.
But my plan is to start a little urban garden on our balcony out of recycled materiel that I'll collect myself.
I have seen it done many times in Southern Africa and Canada and the climate is much friendlier here...