1)i'd check tropical fish stores. ask for the super large bags.
2)keep the growth point healthy by securing the plastic bag within the box to prevent shifting. depending on the size of the plant, you can also put it into a large tube which functions as a canister. (place newspaper lining on the bottom, and on top of the plastic bag containing the plant)
3)packing material: newspaper, or fillers.
4)do not remove whatever is attached to the roots, rather use some LFS and wrap it around the roots and clumping material to protect them. could use a rubberband to secure the sphagnum. (do try to remove as much as you can to the best of your ability though--the plant is less likely go into shock as much if you dont remove all the media from the roots) after that, i would place the bundled roots into the corner of the plastic bag, and then use a rubber band to secure the base of the plant so the plant doesnt move around inside the plastic bag and possibly damage the growth point.