I haven't bought the wood from anywhere, but I am making my own wood floor from red oak that was picked up at a local lumber mill.
measurements are as for off the shelf lumber (1x6 is not exactly 1 inch by 6 inches, you get the idea)
for a 1968 Chevy C10 short step
2 1x4 for the edges
4 1x8
2 1x6 in the middle
looks like this when assembled...