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.
Here's an upcoming option that may work, a Thunderbolt hub. An example, not an endorsement: https://eshop.macsales.com/shop/owc-thunderbolt-hub
A Thunderbolt hub like this likely, but not absolutely, has a USB 3.x controller in it allowing USB 3.x devices to be used with Thunderbolt 1 and 2 hosts as well as any Thunderbolt devices. I'm considering one for my own use and if I get one and I remember to come back to this post then I'll report on how it works. But to get it to work means having a Thunderbolt/mini-DP to Thunderbolt/USB-C adapter and/or cable.
So, a quick summary. A single Thunderbolt/USB-C device can be connected with a mini-DP to USB-C adapter. A single USB 3.x/USB-C device can be connected by a USB-A to USB-C cable. This will not always get the most bandwidth a USB-C device can offer but it is cheap and easy. A Thunderbolt dock is an option, an expensive one that may be overkill. A Thunderbolt/USB-C hub should allow use of multiple Thunderbolt and USB 3.x devices with a Thunderbolt 1 or 2 host. I have not tested this myself but, like with the dock, I recall reading that people had success with this in the past. The hub saves some money over the dock but the dock may have the ports you needed USB-C dongles for and so saves on clutter.
Edit to add: I have tested a Thunderbolt hub with USB-C on a Thunderbolt 1 computer since I first posted this answer and it does provide useful USB-C ports. Even though the hub is Thunderbolt 4 it worked on a Thunderbolt 1 Mac Mini by using an Apple Thunderbolt USB-C to mini-DP adapter and Thunderbolt/mini-DP cable. The hub has a USB 3.x controller in it so USB devices that were plugged in still worked. The way I read the Thunderbolt spec all Thunderbolt hubs with USB-C ports must have a USB 3.x controller for backward compatibility, so this should not be unique to the hub I bought. The nice little bonus of this is I get a 10 Gbps USB controller in the hub on top of the 5 Gbps controller in the Mac Mini. Since the Thunderbolt controller in the Mac Mini is 10 Gbps the bandwidth is limited, I can't get the same USB performance as if using the same hub on a computer with Thunderbolt 2, 3, or 4. The Apple Thunderbolt adapter does not support DisplayPort so any video ports on a Thunderbolt dock or hub plugged in using the Apple adapter are not likely to work. This should not impact the function of an eGPU as they use PCI or USB to produce video.