I'm not sure where to post this so I'm hoping onebag can help. I'm a band director and I carry large music scores (up to 12 x 18) and I'm looking for a bag that I can use to transport them back and forth from work. Ideally it would be less than $200 Canadian. Closest thing I've seen was the timbuk2 alcatraz backback but you can't get that in Canada...
Do you only need to transport the music scores? To and from work as in a daily commute? Or are you traveling long distances (ie flying, taking train, etc) to different locations?
Would a side satchel/brief case/etc style bag work? If it's a daily commute with only the scores and possibly a lunch/other small things a smaller bag might be more useful.
I don't own these bags, but this is where I would start. Decathlon offers decent quality at decent prices, and it's a good chance to try something out and see if it works for you.
I personally have an Osprey Farpoint that I really like. It's 14"x22" and can be carried as a briefcase or a shoulder bag or a backpack. It's quite a bit pricier.
I think you'll want to make sure that whatever you get is relatively square (given your purpose) and that it opens flat. Let us know what other considerations you have, and what you finally decide to try. Nothing beats taking a few scores to a local store and trying a bag in person.