This is of interest to me as well:
I can tell you that the fasteners used are as follows:
1/4"-20 socket head bolts (not hex bolts, there is a difference). 3/4" and 5/8" lengths. Primarily used in the frame. I believe this is what you are inquiring about.
The remaining fasteners are:
M3 screws for the motors and for the underside of the x carriage units to hold the rods in place
#4 screws for connecting the thinner aluminum extrusions and assembling the extruder assembly.
5/16"-18 bolts for anything that involves a 608 bearing, and the feet.
The part I find interesting is the black finish of all the fasteners. I've found that there are fasteners coated with a "black oxide" finish which provides some corrosion resistance. It's not easy to find all the fasteners used here in that finish.
But, if the natural steel color works for you, I recommend boltdepot.com. That's where I got everything. And remember, there is a difference between hex bolt and socket head machine screw.