M&S in Tripoli doesn't sell any food items, however since you care about such organic stuff there are other ways to get them. But this will not probably the kind of food you are used to.
I am not a fan of food globalization, and I don't like the idea of people trying to have the same diet every where, I advocate for eating local all the time.
I don't expect you to find organic cans or cereals in Libya but you can ask for local food that is very healthy but different, after all it is a type of Mediterranean diet.
In shot I will say try to get olives and olive oil from farmers make sure that their olive trees are not treated with any chemicals mostly they don't (mostly they are wild they don't even irrigate them or anything) but good to ask. You can get nice fish that is fresh from the sea for a good price from the fish market. Also you can get a variety of good dates from different parts of the country.
in addition to seasonal stuff like figs, oranges and almonds, apricots..etc, you can always ask friends to get into local farmers and buy stuff from them directly you can even get raw milk if you like! which I can't get here in Canada.
In short my point every country has it's diet... you can exchange the cereals with dates and "Bsisa" for breakfast, and fish couscous instead of pasta for dinner after all it is up to you... :)...