Could anybody tell me what particular post from Lowe's would match this mailbox?

Go down to the end of the isle where the mailboxes are. You will find a pre-made 4x4x84 inch Gothic CA copper Azole, treated deck post part number 461822. Only problem is you will have to make a piece of wood that will go under the mailbox. I made one to just fit under the box with a slight gap at the front to allow the door to drop down all the way. Drilled some holes on the bottom sides of the mailbox. Then used some deck screws to mount the board to the deck post. Don't slide it all the way to the back as the mailbox has a lip the will hit at the top of the vertical post. it needs to be about 1/2 inch forward. Screw the board down. Then using some shorter deck screws, attach the mailbox to the board you attached to the post. Remember the postal regs. require the bottom of the mailbox to be 41 to 45 inches above the pavement. It also needs to be 10 inches back from the edge of the pavement.

I purchased a 4x4x8' treated post from Lowe's stock. They could not cut it down for me in the store, but it only took a few minutes with my handsaw.