There are carpet stores all over Tehran, but nowhere is the experience as memorable, and the price as negotiable, as in the bazaar. There are literally thousands of merchants to choose from selling carpets from luggage-friendly prayer mats to gigantic antique pieces that you'll need to ship home.
Don't worry about finding the stalls – there’s a high probability that at least one merchant will approach you soon after you arrive. Go with it, remembering you are under no obligation to buy.