if you have a minor amount of do-it-yourselfness any decent local bike shop can give you a used bike box for free. you'll need to do a few things like take the pedals off, loosen the headset and turn the handlebars sideways, take the front wheel off, ect.....there are guides to doing this if you use the old google.

i had never done it before and we got 3 bikes to europe and back, plus one intercontinental flight where we bought bike boxes at the airport, no problem.