Check with the airline they are travelling with, they may be able to bring it as excess cargo/baggage. Some airlines don't charge for this as it is medically required, others do charge)
We are considering shipping by mail what Jacob requires before we travel out west. Granted, we are staying in the country as of this point.
I work for an airline, so althought its not something we have actually done yet, its always on my mind. I am somewhat limited, as we travel standby, and cant really bring additional luggage. Plus, too often I have seen misdirected baggage which scares me when it comes to formula!!