I'm replacing my parents' mailbox which is mounted on a 2" steel post. If you have the bracket that accomplishes that and your screws haven't rusted it onto the mailbox, congratulations, you have a unicorn. Evidently only 4" posts are used now and mailboxes don't come with any bracket. I've been to Lowe's a d Wal-Mart so far, a Lowe's employee suggested Amazon. Next up is Ace and Orscheln's, maybe True Value, if none of those work, old box is going back on with new letters until next time I am home.

I'm probably talking about something I know nothing about here, but could you use 2" water pipe and a floor flange?

Something like this:

Could you mount wood to existing post, and mailbox to new wood?

I ended up finding a new bracket that came with the post at Ace, $24 but immense time and hassle saved. The pipe us now two piece aluminium and the frilly decoration under the mailbox is plastic instead of metal. I kept the mounting hardware kit and donated the rest.