Why must 到 be chosen in the following sentence? I just took an EasyMandarin test, and one of the questions is:

飞机票买得___吗？

The possible candidates to fill in the blank is: 了，去，过，and 到.
I picked up 了 but the correct answer was 到.
Now, according to this answer, the difference between 到 and 了 is:

了 and 到/著(着) (zháo) serve different functions in sentences.
了 used after verbs implies a sense of &quot;stopped, finished,completed, or done&quot; on the action.
到 and 著(着) used after verbs indicate the result of the action.

But then why is 到 correct here? It seems to me that the sentence is &quot;Did you purchase a flight ticket?&quot;, which is a completed action, so it seems 了 is correct.
Why is 到 the correct answer and what is the meaning of the sentence?

UPDATE
Hmmm... I still don't get it. It seems that 买得到 as a phrase functions as a verb (but 买得了 not). So in this sentence is 得 not considered a degree complement?
"飞机票买得到吗"  means "Do I have any chance to get the ticket?" (usually ask when you are not very sure whether if there is any ticket left)

"飞机票买得了吗"  means "Can I AFFORD a ticket?" or "Is ticket for sale?" (this sentence is correct in grammar but it is meaningless to ask such a question)