I have a 2012 MacBook Pro. It has a Thunderbolt (Mini DisplayPort) port. I'd like to plug a USB Type-C device into it, since Thunderbolt is faster than my USB 3.0 port. I'm having a hard time finding an adapter with male Mini DisplayPort on one end and female USB-C on the other. Does such an adapter exist?
Use this adapter if it's a Thunderbolt device -> https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter
You will need a Thunderbolt cable as well. That should work for you, it's worked for me.
If this is not a Thunderbolt device with a USB-C port, but is instead a USB 3.x device with a USB-C port then you need a different cable. Like this one -> https://www.amazon.com/AmazonBasics-Type-C-Adapter-Charger-Cable/dp/B01GGKYR2O
If you want to get gigabit speeds then make sure it's a USB 3.x cable and not a USB 2.0 cable.
If the device has a captive USB-C cable then there is no cheap solution. You will need a Thunderbolt 2 dock with a USB-C port, and these are hard to find. It would likely be cheaper to just replace this USB-C device with a USB-A or Thunderbolt 2 version than get an adapter.