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Radioactive elements on the earth come from one of four sources, and depending on the source, they may or may not be replenished.
1 - Some radioactive elements were created in the supernova 5 billion years ago that formed the earth. Theses elements do not replenish. Anything with a half life of less than about 100 million years is effectively gone now and won't come back. The long lived isotopes left from the supernova that are still here include uranium-235, uranium-238, thorium-232, potassium-40, and a few others. They will be around for a few billion years.
2 - Some radioactive elements come from the decay of other elements. In particular, uranium and thorium (from the supernova) decay into shorter lived isotopes which in turn decay into other radioactive isotopes. This produces decay chains starting with uranium and thorium and ending in stable lead. These isotopes will keep being replenished as long as there is uranium and thorium to feed them, a few billion more years. In this sense the uranium and thorium act as reservoirs for these shorter lived isotopes. This includes elements such as radium and radon.
3 - Some radioactive materials are produced by cosmic rays, and are constantly being replenished. These will never run out (as long as earth is here). These include tritium, carbon-14, phosphorus-32 and some others.
4 - Some are artificially produced by humans. This is by far the smallest source. Some are made in nuclear reactors as a byproduct of fission. Some are deliberately manufactured in reactors and accelerators. These will be replenished as long as we continue to make them. These include iridium-192, technecium-99, americium-241, cesium-137, cobalt-60, iodine-131, iodine-135, strontium-90, plutonium, tritium, carbon-14, and many, many others.
I hope this helps.
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