How can i buy an adapter for this?

On amazon, i bought a pair of headphones that were usb-c by mistake. I use iphone and a pc computer, they don't have usb-c plugs. The headphones are usb-c and i'm trying to find an adapter that makes usb-c go to a 3.5mm audio jack. I want one where the usb-c side is "open" and the 3.5mm jack side is "closed" (no hole). I don't really know how to explain this, hope somebody understands.

I've never seen one that goes that direction, they are always the opposite (male usbc, female 3.5mm)

You won't find one. In order to perform the conversion from 3.5mm jack to outputting for USB headphones, you'd need a power supply. It's just not something that anyone expects anyone to do, and so there's no adapter for it!

There won't be a simple adapter, USB C is digital, headphone jacks are analogue. You would be better returning the headphone and getting the correct type

Just google usb c to audio jack and see if what you want is there.

3.5mm plugs for headphones typically do not carry enough power (there's an exception for *some* camcorders' mic jacks that have "plug-in-power", but these are the exception).

USB headphones - whether USB-b or USB-c - can carry power (typically for the little LED showing they are in use and sometimes for the mic and speakers associated with the headphones.

Since the 3.5mm connector has not enough or no power and the USB headphones generally need power, what you want does not exist/is not possible… Unless you add in an external power supply and the whole portability design is out the window.

USB 3.0 male plug adapter with USB-c female in/out.
Up to you how to get the audio stream to it. Might work with a PC, not with an iPhone.
Here is a DIY kit for 3.5mm audio to USB-headphones. Electronics needs power though. Might be too cumbersome with an iPhone.
Here is the Apple Lightning for iPhone cable to HDMI adapter:
Here is HDMI to USB 3.0 converter: https://inogeni.com/product/hdmi-to-usb-3
Here is USB 3.0 to USB-c converter: https://www.newegg.com/...Pages-_-NA