i found the buttermilk at mega mart the centre(al-sadd/muntazah).1 cup 284ml is 13 riyal.i dont know if thats cheap or expensive but i'm just so so so happy to have finally found it i bought 2.thank you so much people!
p.s from my research,laban can be buttermilk substitute but they are not the same.research the internet,you will find out that you can use little buttermilk added to normal milk to make a new cup of buttermilk.leave that new buttermlik for few day,and you can use a little of that buttermilk with antoher cup of milk to make new buttermilk and you can repeat the process over and over again.that's one of the cheapest way to produce your own buttermilk.