I have been building my plane out of Doug Fir. Although there are times
that the wood splits, after a while you learn how to keep that at a
minimum. Good Doug fir is not hard to find but you do have to go
through some stock to find what your looking for. I milled my pieces
out of 4x4's from Home Depot. When it came time to get longer good
pieces, I contacted a company in Oregon to ship me rough sawn stock. I
got lucky that the saleman was interested in what I was doing and hand
picked the grain for me. Also I found that the rough sawn was a better
way to go when milling your own. I wish I would have started there
instead of ending there.

As for Sitka Spruce, stay away from the advertised companies and look
in the phone book for specialty lumber companies. I found a place in
norhtern Illinois that has good quality straight grain sitka at half
the price. They just don't advertise it as aircraft, no liability.
Also, don't tell them that your building a plane.
Lou