grammar - Is は used when asking if something already discussed exists?

If I've talked about a thing before, and want to ask whether or not it exists in another place (or time), would I use "は"?

For example, if I said "In Sydney, I saw a ryokan. Are there ryokans in Melbourne?", would I say "は" or "が" in

シドニーで、旅館を見ました。

メルボルンに旅館（は／が?）ありますか？

By contrast, if I hadn't talked about it before, would I use "が"?

私はメルボルンに行きます。

メルボルンに旅館がありますか？

(The reason I'm asking about this is that I probably got the wrong idea about when to use "が" and "は" because I misinterpreted the grammar section of "Japanese For Busy People I" Unit 4 (page 68 in the kana edition))