I believe the Pandaboard comes with an RS232 connection. But you could also try the Beagleboard. You might want to take a look at the Gumstix Overo line and their various expansion boards, but I'm not exactly sure if they provide what you need. Also, you could try a USB->RS485 converter if you need that specifically.
If you're willing to just breadboard it, or want to integrate it into something else, an FTDI chip + Maxim 1308x is basically what they're selling you, and I don't think they cost half that.
