Scenario: Since my feet are different in size, I wanted to ask the teller at a shoe store if I could buy a pair of shoes that have different sizes (i.e. one shoe is slightly bigger than the other). I am a non-native English speaker and was unable to express my point in one simple, unambiguous question. How should I phrase my question in this case? Thank you very much.
I think that the most concise way of saying this is that you need a pair of shoes not of matching sizes.
But as @Lawrence commented, the most natural approach would be to open with a statement explaining why you need that. There are many ways of phrasing that. You could ask: My feet are different sizes. Can I buy a non-matching pair? Another way would be: My feet aren't the same size. Is it possible to buy two different-sized shoes?