You need to search some planets for them. I've made my own data sheet of locations near my home system, you could do the same. Or, alternatively, if you're in the ALD area of the bubble you could just use my spots - let me know, and I'll give some for you. I do highly suggest going out on your own. I recommend searching High Metal Content planets, as they seem to have the best availability of, well, everything. Generally speaking, that is. There are some bad planets in each system, but for every bad planet there is also a good planet (and vice versa).
Edit: forgot to mention that what you're looking for will be the yellow outcrops with metallic sounds, and the metallic meteorites. Those last ones are way, way better, but also harder to find - so go for them whenever you see them pop up on the wave scanner. Alternatively, if those two are nowhere to be seen, mesosiderites are also a source. But an actual drop is far less likely.