Since you asked about a UART specifically, the CP2102 based USB-TTL adapters output 3.3V TTL which is compatible with the Pi GPIO. They can be found on eBay for less than $2 (a bit more if you want faster shipping). They usually have a USB-A connector to plug directly into a USB port, but there is another style with a micro USB jack that is more convenient if your Pi isn't located near your computer's USB port (uses an ordinary micro USB cable).
Note the micro USB model doesn't have the header installed, so it will require soldering (the USB-A type have pre-installed headers).
Three pins are used: Transmit (TX), Receive (RX), and Ground (any ground pin can be used).
Using the micro USB model above as an example, that would be:
The other connections are not used (although the +5V output could be used to power a Pi Zero, if you're not using the USB power jack
You could also send numbers over WiFi or Bluetooth, and if you are using a Pi Zero (you didn't say), you could use USB Gadget mode.
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?