Depends if it's for diet purposes or because you can't find single cream:) For diet, I use Total 0% yoghurt, which is nice and thick; otherwise, I get ordinary cream and add lemon juice to make it sour cream.
I always find sour cream at the Union Co-op in Etihad Mall. It is good quality and does not cost much! It is on the top shelf close to the Milk! I have substituted with Lebni before as it is a bit thicker than yougurt.
Fatima, Greek style yogurt is a perfect substitute. The best you can find in this market is a Greek brand called Total (the 0% is also excellent, you won't be able to tell is non-fat) or a close second is the Greek yogurt (full fat) of the Carrefour brand. There's another brand but I can't recall the name right now, they have big, tall tubs of yogurt with flavors, cereals etc. Their Greek Style (full fat) is also quite good but a bit sweeter than the other two so it may not work in all recipes.