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))