How do you get a fireplace mantle?

I'm curious to know, how does one acquire a fireplace mantle? Is it a purchase from a furniture store, a custom-made piece, or perhaps a restoration project from an older home? Are there specific considerations to keep in mind when selecting a fireplace mantle, such as size, material, or design? I'm eager to learn the process and what factors play a role in obtaining the perfect Mantle to complement a fireplace.