Even though it looks as simple as it should be found at 100 yen shop, I was looking for it for a long time. But finally I got it at ep-models.com. It is a ring sold as a gate for drone racing, made of thin cloth and wire with an inner diameter of 35 cm and an outer diameter of 48 cm.
Hanging these on a microphone stand, I use them for practicing to manipulate drone with a small toy multicopter. But actually it's hard to fly the aircraft through the rings without touching them. If you do not control it accurately, then it crashes quite easily.