The only way that would make sense is if the communication with the device needed to be done at a higher baud rate than possible using software serial. I'm lacking the information to know whether that is the case.
Anyway, I'll just answer your question. This is the one I use:
https://www.ebay.com/itm/312519249465
Some people don't like the cheap "FT232" modules because they use counterfeit FT232 chips, but they work great for me.
The CH340 modules are a good alternative, but you need to do some searching to find the ones that are in the most useful form and they are a bit more expensive. The most common and cheap CH340 modules have a very stupid form and you'll probably regret buying one of those (I know I do). DrAzzy sometimes recommends one that looks nice, but I couldn't find it. I did find these, which look decent:
https://www.ebay.com/itm/183714223269
My advice is to make sure the one you buy has the standardized "FTDI header" pinout:
- DTR or RTS
- RX
- TX
- VCC
- CTS
- GND
That will allow you to plug the adapter right in to an Arduino Pro Mini, or many other boards that use this standard pinout. If you get an adapter with a non-standard pinout, you would need to use a mess of jumper wires to adapt it to the standard pinout, and some don't even break out all the signals you need for full functionality! I realize none of that is needed for your Uno, but this USB to TTL UART adapter is a very useful tool, so it's worth getting a good one.
Don't buy one of those with the USB A connector. That would be super annoying!
You'll note that my recommendation has male pins, while one of the CH340 ones I linked to has female pins. In the case of the Pro Mini, they come without an FTDI header soldered, so you could choose whether you want male or female. I have standardized on the adapter having male pins because that's the way my preferred adapters are. I notice the LilyPad and Arduino Ethernet (retired) use male FTDI headers. So I'm not sure which is the best choice there, but I would pick one and try to stick with that convention on all my devices.