Where are the spore-containing sori of a fern found?

The spore-containing sori of a fern are found on the underside of their fronds, which means that they are located on the underside of the fern leaves.

A sorus (plural - sori) refers to a group of sporangia (which include spores) when it comes to ferns and fungi. So, when it comes to ferns, they are colored yellow or brown, and located on the underside of the frond - and a frond is a large leaf.