[southwestforests]

I'm not sure what the names of the tools are and which would be what quality, but within the stick and tissue airplane modeling world there are tools available for cutting your own strips from sheet.
FAI Model Supply, who I occasionally get rubber from for my rubber band powered balsa submarines, might be a good place to start looking and asking.
Also, Cleveland Model & Supply Company, who offer a lot of vintage plan sets.
I expect cutting your own strips from sheet would be more economical than buying strips.

And those 2 places might have people who know where and if longer balsa lengths can be acquired. One such place I know about because someone somewhere once said and the name is easy to remember, but haven't used, is named Balsa USA.