Some have suggested that agriculture, with numerous suppliers, and almost perfectly substitutable products, is an approximation to the perfect competition model. This may have been true in some places at certain times, but in modern economies the assertion is hard to prove due to the complex nature of political policies and wealth distribution.
Perhaps the closest thing to a perfectly competitive market would be a large auction of identical goods with all potential buyers and sellers present. By design, a stock exchange closely approximates this, though there is no way to guarantee atomicity (or even approach it). As perfect competition is a theoretical absolute, there are no real-world examples of a perfectly competitive market.