"A lot of people think that those places are difficult to find."
Translation:Molte persone pensano che quei posti siano difficili da trovare.
Maiden and Robustelli agree with you; sono is an error; it should be siano. Spanish is different in this respect, in that you cannot use the subjunctive after to believe or to think but you must use it if they're negated. Italian requires it always, "no matter how strong the belief." They have an example sentence very much like this one.
"Il 78,4 per cento degli italiani è convinto che la delinquenza sia in aumento."
"A Reference Grammar of Modern Italian" (Maiden and Robusttelli, 2000, pp 325, 326)
Maiden and Robustelli are wrong there. Credo che can be followed by the indicative mood if it has the meaning of probably:
Credo che mi annoiavo e anelavo il momento che la giornata riprendesse.
I think I was bored and longing for the moment when the day would begin again.
—Cesare Pavese, Storia segreta, in Racconti, Turin, Einaudi, 1960, p. 485
The indicative mood is even obligatory if credere or pensare are in the imperative mood and not negated:
Creda che sono/*io sia veramente mortificato.
Believe me, I am really sorry.
The indicative mood is also the norm after expressions like dar a credere and far credere.
And let’s not forget that you have no choice but choose the indicative mood in the future tense.
Also, why did they pick an example with essere convinto, which has nothing to do with credere and pensare and traditionally takes the indicative mood?
"Siano" is 3rd person plural for the verb "essere" (to be) and "Stiano" is 3rd person plural for the verb "stare" (to stay). [Refer to https://www.italian-verbs.com/italian-verbs/conjugation], Present Subjunctive tense.