I just fixed the same thing on my '64 and I had to make the bottom of the post, I couldn't find it anywhere. It's pretty easy to make if you've got snips and a welder.
If you are restoring the whole van yourself I would suggest picking up a metal shrinker (~$300) that will allow you to make that part perfectly as well as the lower section of the wheel arch shown in your pic, and many more areas like the lower inside structure for the side doors. You can buy the whole wheel arch if there is any more rust anywhere on it, that's what I did to mine.