Where to find a 3x3 post?

[Luck_of_the_Draw]

Makita hand planers are my brand of choice, but again,not very effective for planning 1" off of 2 sides of a 4x4... table saw is much easier and you'll use it more in the long run

4x4 = 3 1/2 x 3 1/2

Run it through a planer for furniture