appliance shops, scooter/e-bike stores, furniture places.....all have large boxes.

box cutter and duck tape is all you need.

it's actually preferable to make your own box, instead of trying to fit a full-size
touring bike with racks/fenders in a small-as-possible bike box.

print off the sporting goods/oversize box limits for YOUR airline.
build a box within a couple inches of the maximum, sized to fit
your bike.

i usually fly jetstar or tiger on A320's, allowing (max 32kg) to 1.90 m length.

i make the box wide enough to add some padding on both sides of the rear
rack still on the bike. long enough to keep keep the rear wheel and rear rack
and fork and fenders on. just remove the seat and front wheel, then ziptie
the bars to the toptube. re- and dis-assembly at airports quick and easy.

never had a problem at the counter. box is too large to take as carry-on
on the bullet trains, but will fit in the station-wagon style taxis.