Around here 9 inch center sections are quite plentiful and every ford guy has a few extra punkins around. Even the junkyards have school busses full of them already removed and if it isn't an LS they are dirt cheap. I have a couple 28 spline limited slips under my bench for the 29 PRU hotrod, and I've given away maybe 4-5 opens in the last year that were 3.00, 3.25. 3.50s and so on.
Network, ask the parts store guys, you'll find the Ford guys with the stuff you need.
I will say that every 67-72 I ever pulled was a 28, even though some were supposed to be 31 they must be rare. That's not including a few Dana axles in certain 67s but that's a whole different unrelated deal.