No. In this case, 「半日のツアーはありますか。」 is better.
The first thing you should learn is that the 「は」 particle can also be used to indicate the subject. Try searching in this site and you will find many answers that describes the difference between 「が」 and 「は」.
This 「は」 is the topic marker. The topic of this sentence should be (the existence of) a half-day tour, so 「は」 fits here.
On the other hand, 「が」 is not appropriate because it sounds like introducing a certain half-day tour into the context, though the speaker is not sure it exists.
So if we want to say "they have a half-day tour", 「半日のツアーがあります」 is better than 「半日のツアーはあります」, becuase the former brings into a half-day tour that we already know into the conversation.