We used to use the A4 Document Wallets. They were cardboard-y and light, we could buy them in a set in different colours (red for Christmas, blue for contest etc....) but they tend to show their age.
There's apparently a similar concept made out of plastic woth a velcro buckly.... :? Or even some of my little kids have plastic ones witht hose little buckles like on kids raincoats.
The popular choice is the Display folder. Two plastic covers, a 50 godzillions spiral centre with 30-50 inserts. The spiral spine coems apart to add extra inserts. Catch there is the spiral breaks quite easily.....but everything has a negative, right??
We currently use an A4 Ring binder. Yes the heavy, awkward shape can bite the big one at times, but ours have a plastic layer on the front so you can slide a title page onto the front cover. Ours look verrrrry pretty!!
Finally, a more expensive option is the professional folio. A plastic-over-A5-cardboard design, it has 2 plastic slips (one per side) to insert music. Then they fold in Galf to look like an A4 book. They are capable of holding HEAPS of music, a good weight, and shape. YOu can get them professionally labelled and such too. I think in Australia they're $30 EACH(!!!!) through Allans Music (no relation - different spelling!!).