Arduino Ethernet board serial connection to PC

That page tells you what you need to know to program the board: The 6-pin serial programming header is compatible with the USB Serial adapter and also with the FTDI USB cables or with Sparkfun and Adafruit FTDI-style basic USB-to-serial breakout boards.

Look here: https://www.sparkfun.com/search/results?term=ftdi&what=products

And here: https://www.adafruit.com/index.php?main_page=adasearch&q=ftdi