Any of the horticultural suppliers will have this, sometimes they call it "yabanci ot ilac" which translates to foreign weed poison.
However if you do want large quantities of sodium chloride, nice and clean sea salt type for food or pool purposes try Sevikoglu in Dalaman, Ortaca or Dalyan I have recently bought 5 tonnes which they had and could deliver next day.
As a weedkiller, Sodium chlorate persists in the top few inches of the ground for over a year and will succesfully kill off relatively shallow rooted plants. It is however potentially explosive, and so a risk prior, during and for a while after application. It also 'creeps' sideways from where it is applied and so can kill off plants perhaps not intended to be killed.
As PASH says 'Roundup' is a weedkiller widely available in the UK. Its active ingredient is Glyphosate, which in itself is not a poison. However, when absorbed by a plant, the plants own chemistry converts it into a poison and so it is translocated (transported) throughout the plant killing it 'down to the roots'. It will only kill plants it is watered (better with a v fine spray) onto and so does not stop new weeds germinating - but your plastic membrane will.
Many other really effective weedkillers have in the past few years fallen foul of the 'Health & Safety' brigade and been banned under the Control of Substances Harmful to Health (CoSHH) regulations.