If you buy quarter grain wood its usually a lot straighter than other grain patterns! You could try spraying it with water or alcohol and putting it under heavy weights for 24hrs ? I usually find that as soon as I get the wood out of model shop it tends to go all shapes If you buy two pieces with a similar bow you have the curves for your fuselage sides! I've done that before

Cheers

If you can stand the smell and the wife doesn't throw you out,  Use Amonia.   Doesn't take much in a bath tub to get it pliable.  Have a straight flat surface to lay it on with another flat surface to put on top.  Let sit for a week or more.   I vaguely remember  that using Amonia will releive all the wood fibers.

The original question was simply about a warped sheet of balsa, not a sheet with edges that are not straight.  So, I think Doc wins, Ty loses.    

If you're looking for a straight plank for a profile fuselage, cutting a sheet into strips 3/4" wide and alternating the bow in them when you glue them back together would work well. You'd need to have a table saw, or access to one. Better yet, build up the profile fuselage with stringers and warren truss diagonals, or a foam slab for fill. I like this approach, because you can easily taper the profile fuselage from TE to rudder post, putting stiffness where you need it and "adding lightness" where you need some o' that.    Steve

Amazing how easily feathers get ruffled.  An article I read years ago in one of the RC magazines was about forming wood.   Especially balsa wood.   The guy says he used pure amonia to soak wood in.   He also stated that balsa would become like a thin sheet of paper.   Was easily molded around objects.  Not compound curves with out cutting slits.   The real statement was when he stated, "The amonia releases all the strsses in balsa, if left to dry on its own would curl up.   Had to be weighted down flat or formed around and object.   Once dry it would hold its shape.".   I have done it on foam leading edge bucks for molding the balsa leading edge.   Once it was dry it only took a few peices of tape or pins to hold in place.   So everybody has their way of doing things.   Some I have tried and some I have ignored.   Be we all have ideas that work.   

No need to use ammonia on balsa.  The function of ammonia is to soften the compounds lignin and subrin.  There is little or none of these compounds in balsa. I use water with a little bit of Simple Green to cut any oils and wet the cell walls.  Warm water would help, but I've never used it.