I hope I am correct in understanding that "Iran's English coursebook" is the answer to my question "What is the source of your quote?". If so, can you tell me the actual title of the book and its authors, and whether it is available online? Thanks.
As to other errors, they vary in degree, but there is something wrong with almost every sentence in the paragraph.
It is very difficult to find your favorite food when you travel.
This makes the assumption that the author knows what your favorite food is and where you are travelling. One of my favorite foods is chow mein, and I can find fairly good chow mein in almost any city in the United States, but it's more difficult to come by in Costa Rica. Good authentic Mexican food is easy to find in California and much of the rest of the western United States, but not so much in the eastern part of the country. If you are using "Iran's English coursebook" as your source, perhaps you are partial to Persian cuisine. I can only say that I wish I knew more about it. I'm not sure whether I've ever had the pleasure.
Sometimes it is even difficult to find healthy food.
Okay, this might be the exception. Grammatically, at least, there is nothing wrong with this sentence.
Besides, the prices of foods may be so high on the roads or in airports.
We've already talked about "on the road(s)". The word "so" doesn't work in this context. The entire phrase "the prices of foods may be so high" should be reworded as "the price of food can be very high".
Long trips may make you tired and weak and this can increase the risk of illness. So you should eat well while you are traveling.
This would be better reworded as:
Long trips can make you tired and weak and this can increase the risk of illness, so you should eat well while you are traveling.
The word "may" is not incorrect here, but "can" seems more natural to me. In any case, the author is using "can" and "may" to mean the same thing, so for the sake of consistency, one or the other should be used in all instances in this passage and the previous one, rather than a mixture of the two. The word "so" is used as a conjunction here, as it is in this sentence, so it either needs to be omitted, or the passage needs to be written as one sentence, not two. Incidentally, both "traveling" and "travelling" are accepted spellings. Just be consistent.
Ah, another question. Can we use "in the road(s)" here instead of "on the road"?
I can't think of a normal example where I would ever say "in the roads". We can say "in the road", but it doesn't have the same idiomatic sense as "on the road":
I had to take a detour because there were several dead cows in the road.