Forget Home Depot and Lowes. In my area no self respecting contractor does business with either of these places.
What you need to find is a good old fashion lumber yard. They are around, but they cater to the professional builder. No REAL contractor is going to waste his time picking out 300 2X4's at Home Depot. They just call their lumber yard and they have it delivered to the site.
These places are a GOLD MINE for referals. What I would suggest is find one near you and go down there an open your own account. Have a salesman show you around and let him sell you. Tell him your a real estate investor and you have some projects coming up that you MIGHT consider using them on. This single phrase will light off that saleman's "go get it" gene, then ask him if the he or a manager can recommend a few GOOD reliable contractors for your project. Tell them you'll buy every stick of lumber from them as long as their competive, and you'll see what REAL customer service is like.
Home Depot and Lowes are for Homeowners. Don't ever forget that.
I go in there, I even buy things there. But when it comes to BIG purchases like a house worth of lumber, sheet rock, roofing, or special order items........ go where the pro's go.