Heating Register Circular Where Can I Find Circular Register Vents For A House Heating/cooling System Instead Of The Standard Rectangle?

Where can I find circular register vents for a house heating/cooling system instead of the standard rectangle? - heating register circular

I build my own apartment and I all the features you do not want. I was back with friends at a party and saw the house used in the circular holes in different parts of the system for heating / cooling. They provide a uniform temperature on the standard recording, unlike the rectangular openings. Where can I find those round holes? And what is this kind of system is called? Thank you in advance for the help.