A proper onsen-tamago is, of course, prepared by dipping it into a real onsen spring (not the bath, but the real steaming hot spring) for a while.To do this at home, all you have to do is to find something that can maintain a fairly hot but not too hot temprature for a long enough time.
There are onsen-tamago-makers available, but you don't need this. You can put a raw egg into a thermos full of hot water overnight, and you can have a nice onsen-tamago in the morning.
Or what I do is to put a raw egg inside a rice-cooker that is warming rice. What I mean is, don't put the egg in as you are cooking rice. But after the rice is done, most modern rice-cookers automatically switch to "ho-on" mode where the cooker keeps the rice hot. Put the egg in there. Keep the egg in for about 40 minutes, and your egg will be perfect. If you wet the egg just before putting it in the cooker, it will prevent the rice from sticking to the egg. If you leave it there longer, it turns into a tasty hard-boiled egg, btw.
As for poached eggs, I've seen it being made countless times on Japanese TV cooking programs, and I tried to do it myself, but noticed that it's much easier to make an onsen-tamago.