where do I get the base to attach this post box

Lowe's sells a standard base plate in the store beside the mailboxes for a few bucks. It is 2 inches to narrow to fit this one snug. Any scrap of 1x8 or even 2x8 could be trimmed to fit.

I had a large piece of scrap wood from recent house renovation. Otherwise, it would have been back to Lowes.

can use 2 x 4's-any hardware store-call the Post Office to make sure you put the box at the proper delivery level (depends on walking, mounted or rural route delivery)

I'm going to take the standard mounting board, cut it in half, install it long ways on post.