Rolled or flat? If rolled, I use Uline triangular tubes, specifically the 24x3" variety. They're cost-effective, sturdy, easy to store, and easy to pack and unpack. Never had a complaint about them yet.Nill
I also use Uline supplies. I ship flat up to 16x24, rolled above, ...
Where have you all been getting materials to deliver unframed larger prints (16x20 and up)? I don't need anything to hold up to shipping, but for basic transport and hand delivering prints.