Nearly any seed will produce large amounts of these. If you are looking for an easy way to find them, try seed "404" and start hitting the gravel when you spawn. You will cause it to collapse and open up a large cavern.

Well there is a seed called "worstseedever" and all that is,is a place where you can find a dungeon right in front of you.

There is a seed called Buncus and your world must be named Buncus and you wonder around and find can find clay.

The seed i used was called Minecraft there are several big caves which hold a bunch of minerals i went down and came out with 4 stacks of iron ore its not that hard to find