Dave,

Your local hobby shop.

You don't want to buy the sheets of endgrain balsa (like CST sells) for the spar. It is oriented for compression in the wrong direction (across the thickness).

I've disected a LOT of wings and what almost everybody uses is 1/4" or thicker sheets of balsa that are cut and glued. Some use Rohacell in the outer part tip part of the spar to save weight. The compression load is the highest from the top surface to the bottom surface, and that is the direction the grain is usually oriented in. If you need 4 spars, just cut 4.5" or so sections of 4" to 6" wide balsa and glue them together edge to edge. Then cut the spars.

Some builders do use laminated spars so that they get compression strength both vertically and along the length of the spar. There are usually 3 layers with the outsides being vertically oriented. The layers are typically 3/32" to 1/8".

Some builders cap the spar with CF unidirectional laminate before it in installed into the wing. Some put the CF laminate in the skins, and some do both.