Huge backpack and Itoya art portfolios if you get a lot of prints. Holds them flat. Don't have to worry about straightening them back out.
You may be able to find one cheaper if you google around, but they get the job done. If money is no object to you, check out the Baroque portfolios. They are well built and durable (not that the itoya's aren't), but they cost a LOT more and there is often a wait time before they have enough to ship out.