If one chooses to cut their own stock, most of the spruce and firs will work but be careful to select your boards carefully. Douglas fir is an approved aircraft structural wood for spars and longerons and you will want to ask for "C" grade. Its 25% stronger than sitka spruce and only about 12% heavier so its a good tradeoff where a greater margin of safety is desired or weight is not a concern over strength.
Although suitable white spruce from the northern states may be found and cut by those who supply this product, the grading process is nowhere near as stringent as for Sitka used for full size aircraft. Of course the price reflects that. I can buy a log of white spruce for $10.00 for cutting myself. That same log in Sitka from one of the above manufacturers would be five times that amount and unless I visit their milling facility and pick the log myself, until it arrives I still don't know if will meet with my approval.
Be aware that the quality of white spruce often called "aircraft spruce" by model suppliers is not graded in the same manner as true aircraft MILSPEC wood sold by Wicks Aircraft Supply and Aircraft Spruce and Specialty.
A better choice might be to work with quality C grade douglas fir which is readily available at lumber yards. If one's local lumber supplier stocks it, you can look at the boards and select your own, they are already kiln-dried, straight-lined and surfaced, and one can personally grade each piece before paying for it and taking it back to the shop. And only a table saw that is correctly dialed in and simple small-stock-ripping jig are required to get good parts. If one plans out the cuts, there will be very little waste and a $30.00 2x8x12' will yeild about 50-70 good spar caps/longerons/stringers up to 72 inches in length or longer with about 2 hours time at the saw.
If one doesn't have the tools for cutting their own stock or should decide to save time you can purchase stock from a hobby supply. Order several more than what you need and inspect each for cracks, grain runoff, and orientation and flex them to make sure there aren't any hidden defects. discard those that don't pass muster and use them for non-structural members or fillers.
I generally choose a 1x8x12 and hand-select the piece by first checking for warps, twists, and cupping or crowning since I will have to straight-line one edge and rip the final parts on the table saw. Next I inspect both 5-1/4" faces for defects such as knots or pitch pockets and then examine the 1-1/2" edges by looking down the full length of the plank I looking for straight and tight growth ring spacing, 1/8" or less, and zero runoff of grain from one end of the board to the other. Usually the best boards will meet this criteria and I can usually find one or two boards that will work from the stack.But even if one or two growth rings run off in eight to twelve feet or knots are near one edge, a plank can be acceptable and I can work around these defects. A1x6x12 foot C grade fir board costs about $30.00 at my supplier
SAWING YOUR PARTS
Quarter sawing or rift-sawing will yield the best strength for aircraft structural memebers. Quarter sawn is the name given to the grain orientation in which growth rings are perpendicular to the wide face of the plank. This is simple to accomplish with C grade fir since most is flat-sawn, each part ripped off a narrow edge is now a quarter sawn part. With carefull planning of each cut, its quite easy to get every part made from this piece to be quarter sawn as well.
With the availability of low cost carbon fiber one can laminate it to spar caps and other structural members and have bullet-proof spars. The main thing with any spar system though is quality of fit and good adhesive joints: find or make a good clamp for getting the best contact and adhesion with your shear webs to the caps primarily, and take extra care to ensure your sheeting is well cemented to the cap-web box and every inch of rib edge and you'll be fine. If you want extra assurance at the root and joiner box, wrap that with kevlar thread.
Although carbon is great its completely unnecessary in building vintage scale model sailplanes. Just remember that aluminum and all-wood sailplanes flew just fine for decades, and many of them are still flying. Those that are not, didn't die of spar failures but more likely pilot error, lack of maintenance or being left out in the weather to slowly rot away.
And of course this another great thing about building from kits. If you take your time and select and fabricate your wooden parts with care, not only do you have the satisfaction of building it yourself, but you know the quality of every part and assembly in the airframe and have a durable well-flying ship that will last for years and years!