Why must 到 be chosen in the following sentence?
"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)