I would also consider Doug fir to be our local substitute and I often buy the bigger cross sections of building material and resaw - I have found the quality is a little better in 4x6 stock. Challenge is that most need a few months to dry out and still end up costing a little over $3/bf.  I would also price out 4/4 beech from hardwood industries as a comparison (and whatever hardwood is in their bargain pile).  In the end, it might be a small cost to upgrade to hardwood and it makes a much better bench.

I think I saw some at Crosscut a few weeks ago. Furniture grade likely, not cheap. I too am working on a very modified version of this bench and went with 2x12x10' from Home Despot (Doug Fir, not SYP). As mentioned in the book, these are typically less nasty than 2x4 or 2x8 studs and usually less picked over. I have had them in my garage for about a month now.  I plan to start cutting them down to 5' sections and maybe even ripping them down if they seem dry enough. 

Honestly though I'm likely way over my head.