If you don't mind spend a bit of money (though it is worth it IMO) look at Frost River (esp. isle royal or isle royal jr.)
If want a bit more DIY and modding then surplus is the way to go. Depending on your skill with needle and thread, waxing a pack (should be pretty easy, though I have never done it), and just your general know how on canvas packs I am sure you could make one out of surplus.
Of course if your really skilled you can make your own pack from scratch, just buy some canvas and go to work.
Best of luck.