Roof of a house
A basic camping tent, a house roof, and the iconic Toblerone box.
Pup tent or lean-to
A real life example is a Toblerone chocolate bar which is in the shape of a triangular prism.
If you asking where you'd find them in the real world - Binoculars and telescopes are two examples.
Some of the pyramids in Antarctica resemble a triangular pyramid.
any box of tampons
A solid brick is a rectangular prism. (this one example of many)
A cereal box or a t.v. [not flat screen] or a book.
A square box. A special case is a cube or playing die.
The 4500 year old Great Pyramid of Giza, Egypt. This real life example is also the last remaining of the "Seven Wonders of the World" in existence.
A gaming die.
A building would be a perfect example. If you simplify the details of a building, it becomes a rectangular prism. If you specifically mean a "square" prism, that would be a cube, an example of which would be a Rubik's Cube or a few dice.
An Amicelli box! THIS IS NOT TRUE IT IS HEXAGONAL!!
A foreign brick.
A fjord is a real world example of a fjord! They exist in the real world.
Perhaps a beehive has a number of it.