My attempt is:
Preferiría un asiento de pasillo, si disponible.
Is there a better translation, one a native speaker is most likely to say? I would love to know what the most commonly heard sentence would be (even if it's not "grammatically" correct but used in the streets), especially in Mexico.
What if instead of "if available" I had to say, "if one is available"? This sentence:
I'd like an aisle seat if one is available.