I usually buy this hardware from aerialpixels.com because they have all of the specific sizes but I would go with steel for the longer screws(12mm and 14mm) as their aluminum hardware is the softer 6061 aluminum and not 7075.
4xm3x12mm cap head screws
4xm3x14mm button head screws
4xm3x1mm aluminum spacers
8xm3x6mm cap head screws
4xm3 lock nuts(not flanged)
The following standoffs can be taller than 20mm if you desire but the frame is designed for 20mm and the quad flies better with a gopro with 20mm.
2xm3x20 round standoffs(must be the plain round to fit in the 3d print)