It is permissible to deal in souvenirs for tourists, so long as that does not include anything haraam, because the basic principle is that selling is permissible, as Allaah says (interpretation of the meaning):
“Allaah has permitted trading”
So if the item being sold is not haraam and is not something that is used for haraam purposes, then there is nothing wrong with selling it or dealing in it.
Haraam things include: selling alcohol, statues, clothing with images on them, and selling things that venerate the kuffaar such as their pictures, flags and symbols, or anything that helps them or evildoers to engage in their evil and falsehood, such as indecent clothing, greetings cards for haraam festivals and selling things which are proven to be harmful such as tobacco.
And Allaah knows best.