A good craft store usually has a wood section, including balsa wood. You could try Michael's as a start. Even smaller hobby type stores should carry this.

Try Small Parts, Inc.

Hobby shop (toy trains, RC airplanes, etc.)

Yeah dano, that was my first thought, but I can't seem to find one near where I live, which is a bummer.

(To tell you the truth I sort of figured that hobby shops weren't really around anymore. Although now that I've done some googling, I am reassured of their continued existence.)

Exactly what redvixen et al said. A hobby shop will have balsa for "real" airplane kits and for the tailfins of model rockets. A craft store (like Pat Catan's in the States) will have it for... whatever.

Unless it's for a science fair project (exhibit: The miracle of balsa wood), can you tell us more about the project requirements, and we can brainstorm alternative materials? What are the important features? Dimension? Thickness? Flexibility? Opacity? Workability? Will picture-framing matte board work? Whatabout fabric stiffened with white glue? How about a piece of formica? What about wood veneer sheets? etc.

(in case you hadn't guessed, this is the kind of problem I like...)