David Feldman in Why do Clocks Run Clockwise? posits that 1) the boots may deter coyotes. 2) It's better to stink up a fencepost than a house. 3) Some people just do things.
But forget about the boots. Why do people put caps (coffee cans, finials, can ends, and hub caps) on fenceposts? Because less water (and snow) on the ends means that the posts don't soak throughout and last just a little longer. Then again, the third explanation above also makes sense.
Pp
You can find metal fence posts at home depot or your local hardware store. They carry the posts you will need to support the metal fence or made of metal itself.
How many posts you need for a fence 54 x 54 depends on how far apart you wish to place the posts. Usually posts are placed every 6 - 8 feet, but stallion paddocks frequently have posts placed every 4 feet apart. It's not clear to me how many linear feet your fence is, because the question is written as if the fence is 54 feet high and 54 feet long.
dont buy them
a fence
The strong or strongest part of a fence line are its posts. Posts are generally placed partly into the ground for permanent fencing. Posts provide structure for the entire fence, whether using slats or chain-link in between the posts. Even wood panels used instead of slats are typically weaker than the posts they are attached to in the fence line.
You can purchase plastic (most likely vinyl) fence posts online from such websites as Buy Vinyl Fence, Buy Direct Vinyl Fence, Lowe's, Penn Fence and Home Depot.
dont buy them
fence posts
Fence posts.
21 posts.
5 vertical posts are needed. I---I---I---I---I
If you want your fence to last for a very long time I would definitely recommend cementing the posts in.