Those who are travelling to Saudi Arabia by air intending to perform Hajj or Umarah have the following two options regarding where to wear Ihram:
Either to wear ihram in the airport of their country or;
To take it with them into the plane and ask the plane crew or stewards to inform them before the plane passes over the nearest miqat (a place where hajj/umrah pilgrim must wear his ihram at). They will definitely help them as they are accustomed to these situations.