When I am learning Japanese I encounter this sentence. I think that the particle が should be always used, but は was used instead. Could it be a typo? Or something I have not learned yet?
In other words, should this sentence be :
半日のツアーがありますか。(Do you have any half-day tours?)
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.
I am not sure because there isn't context, but I think that は was used to indicate a contrast between half-day tours and whole-day tours. The は particle, other than indicating the sentence topic, can be used to make a contrast or to put emphasis on one thing instead of another. Example:
「全日のツアーがあります。」 「半日のツアーはありますか。」
Translation:
"We have whole-day tours"
"Do you have half-day tours (instead of whole-day ones)?"
For more information on the usage of が and は see here and here.
