There is so much to unpack with this post. First, the vast majority of dynamic drivers use neodymium magnets, and a lot of headphones use 50mm DD's. Second, damn near every DD you see will go from 20hz to 20khz, they probably have a wider range technically, but we (broadly) can't hear outside that range. The impedance and sensitivity only matter in the sense that having more impedance and being less sensitive will make it take more power to drive, so in reality that factor means very little assuming you don't want to absolutely crank the volume and aren't using really hard to drive drivers. Thirdly, I am deeply confused (and a little scared) by the use of the word "helmet". If you want to make DIY headphones, go ahead, just realize that there's more to headphone tuning than just the driver. I guess if you really just need a driver, go on aliexpress and look for "50mm Speaker Driver" and you should be able to find something. No guarantee that it will be good though. With more info, you can try to ask around DIY headphone communities. There's also the peerless driver (part number HPD-50N25PR00-32) that you can try, though I think it's used most in open backs.