Adaptor for old flash drive to connect with iMac

You will want a USB-A (female) to USB-C (male) adapter. You would then plug your flash drive into the USB-A end of the adapter, and the, connect the USB-C end to your Mac.

You need a simple USB-C to USB-A adapter.

For some examples:

USB-C to USB Adapter - Apple

https://www.target.com/p/philips-usb-c-to-3-0-usb-a-female-adapter/-/A-79804275

https://www.amazon.com/dp/B08Z768LCW/ref=sspa_dk_detail_1?

I'm assuming that these flash drives that you are referring to use USB-A connectors ... correct?

If so, you can find numerous examples of USB-A to USB-C adapters that will work.

For example, here's some from Amazon.

An USB-C to USB one.

(246305)