Haven't used fir for this application, but from my perspective there is a lot of work that goes into making the bed pieces, assembly, finish etc.. Given the time and effort involved it would boil down to deciding if the tradeoff to save a $125 or so bucks would be worth it. My guess is somewhere nearby there is a speciality lumber supplier that sells mostly to cabinet/furniture makers and building contractors. My experience has been they will sell to hobbyists too and their prices are much better than a lumber yard or big box vendor. Their wood quality is also superior.
