Sitka spruce has been the choice of the aircraft industry for nearly a century.
One source for high-quality spruce is:
http://www.aircraftspruce.com/catalo...esparstock.php
You can get it to the thickness and length you need, and rip the width required for your particular model application. For example, ripping a half-dozen square spars from 3/8" x 3" stock would cost about $3 or so per 48" 3/8" sq spar. A bit more than basswood, I'm afraid...but not prohibitively so. Of course, continuous grain along the length of the spar is vital, and the member must be oriented with the grain vertical for maximum strength.
Although basswood is suitable for spars because it is relatively light, has nice, straight grain, and typically is resistant to warping, spruce is somewhat stronger and more elastic than basswood. It can flex more without failing - which is what you're looking for in a spar. It is more resinous than basswood, so you should plan on gluing with carpenter's glue (not CA) and reinforcing with shear webs.
For an exhaustive study on the mechanical properties of wood, check the USDA Forest Products Lab at:
http://www.fpl.fs.fed.us/documnts/fp...tr113/ch04.pdf
A good example of spruce in a well-known model is the Sig Kadet Senior - where spruce is used in combination with balsa on the inboard section of the wing, where stresses are greatest...then balsa spars continue outboard.
Best...
pjw