I am not sure if you will need one. You ordered the Axon2 which you connect up to your PC with a USB cable. Also with the Sparkfun XBee starter kit, I am pretty sure it uses the USB XBee adapter to connect up to the PC.
You can also program your Axon2 with a hardware programmer, such as the AVRISP2. But again I think most of these are now USB based.
But if you do get an USB to serial adapter, check around. I know for a fact that for example don't work well with the Lynxmotion SSC-32 board and likewise the Basic Micro Basic Atom Pros (Been there). I am currently using one by BAFO which is working great. However at first I had problems with it, until I started using a powered USB hub. After years of people having problems with USB cables Lynmotion stated selling one which uses the FTDI chip set. Not sure if Trossen is selling one or not.