This was my list for the lumberyard. Just went yesterday. I haven't cut it and installed yet, so use at your own risk.
. I bought #2 white pine and used the dimensions from the gmcpauls diagram to determine the boards I needed:
1x8-8 (2)
1x8-10 (6)
1x6-10 (2)
10 boards total and it was less than $100 for it all.