If you can't find couscous there, I would next try the ethnic food section. Now, I have seen some stores group ethnic foods all together or separately in different aisles. For example, I have seen latin foods over by the taco kits and japanese foods over by the soy sauces in the ketchup aisle. I have seen couscous stacked next to jewish/kosher foods and also by German foods.
Still can't find couscous? Try the pasta aisle. It really shouldn't be there because couscous isn't really a pasta, but some supermarkets think it is best suited for that aisle.
Finally, if all else fails, try the internet. You can find a ton of grocers online who sell couscous and at competitive prices. Don't give up, though! Couscous is a delicious food and you really don't want to miss out!