If you are asking for strong expired domains from a specific very competitive niche like finance or insurance or some shadow niches where black hat SEO is very common, then we are sorry to disappoint you but such names do not exist with very rare exceptions. They are certainly not enough to build a PBN (Private Blog network) for your SEO.
Link building is SEO. SEO is marketing. Marketing is for making money. So Link Building should be understood in the economic context of making money or at least not spending too much. If you find a way to make link building for free or even make money, it’s a win-win situation, is it not?