I use a plastic bag. The bag was wide, so I cut and taped it to make several long thin bags. Once the poster is inside, you can use tape instead of rubber bands to keep the roll tight, so it can be easily removed from the tubes.
The bag I bought at micheals, it was for wrapping gift baskets. I found it by the archival photo sleeves. Also, the bag will keep the poster dry, if the tube gets wet while shipping.