"Where can we buy a phone card?"
Yes, though the measure word in this case may be 张 zhang for a card. I am wondering why many of these exercises omit measure words.
As to your first question:
No, it can't. It would be a different sentence, since:
哪儿/哪里 = where
那儿/那里 = there
Some extra information
A good way to differentiate and also learn up a series of characters would be to focus on the 部首（radical）. Here it is 口字旁 and it is typical of many "question-useful characters" e.g. 呢、吗、哪
"exclamation-useful characters and onomatopoeia": 吧、呗、叩、呜、咳、呵、唉、嗨、咕噜、咚、咔、哗啦啦 and so on.
Further extra information
Since 口 means mouth, it follows that characters with it as the radical 口字旁的字 would be related to the mouth, although functions of it may have its own 部首，e.g. speech 言字旁 where 言 refers to speaking and related things.
As to your second question
Just add 吗 at the end, or the more usual order putting 买电话卡 at the end: 我们可以在那儿买电话卡吗？ would mean "Can we buy a phone card there?".
Another variant that brings to mind would be: 我们（可不）可以在那儿买电话卡？ and this would not need 吗.
Changing 那里/那儿 (there) for 这儿/这里 (here) would probably be more useful irl