I haven't read all of the posts in this thread, but I know that straight lumber in any size smaller than 2X3 is getting progressively harder and harder to find.
I believe that all lumber is straight as an arrow when it's cut to size. The problem is that it's cut to size before it's fully dried. Wood cell wall shrink in thickness as wood dries, and that's what accounts for the twisting that happens when the straps are cut on a lift of wood and the lumber is allowed to move.
Probably the best solution is the one you chose; buy a larger piece of lumber and have it ripped down to size. But, even that option requires that the lumber you buy is relatively straight to begin with, and that can be a problem with lumber sizes smaller than a 2X3.
You can prove to yourself that the wood is being cut to size when it's still too wet by taking a close look at the 2X12 joists at your local lumber yard.
each and every one of them will be split at both ends for anywhere from
12 to 24 inches. The reason for that splitting is because moisture in the wood both is absorbed and evaporates 15 times as fast at the end grain than it does across the grain of the wood. So, the ends of the
2X12's are going to dry fastest, and when they do the wood at the ends of the joists shrinks more than the wood further from the end. The result is that the wood at the end of the joist splits to accomodate the shrinkage in the wood cell walls there.
Canada has an "export grade" of lumber that is prepared specially for export to countries like Hong Kong and Singapore that need lumber for construction but don't have forests or a forestry industry. That wood is dried for much longer AND the ends of the lumber is painted with oil based paint before it's cut into 2X12's or whatever. That paint prevents the evaporation of any residual moisture in the wood through the end grain. Consequently, any residual moisture in the wood (above about 16% which is what wood will typically dry to indoors) has to evaporate across the grain of the wood. The result is that the lumber dries evenly all the way along it's length with no faster evaporation of moisture at the ends and hence no splitting at the ends. The longer drying time and the additional preparation make that wood more expensive to produce, but it also sells for much more at it's export destination.
In a perfect world, all lumber would be made like export grade lumber.