For my workshop, I was planning on using 2x12x20' Douglass Fir at 12" on center to span the second floor. The problem I am having is that I can't find anyone that sells Douglass Fir in that size. I have only found Spruce and "framing lumber in 2x12x20'

Do I need to worry about the difference between Douglass Fir, Spruce, and framing lumber? Will any of the three work?

You can see the plans at: http://www.theworkshopproject.com/plans.php

Thanks in advance.