I'm not in the USA, but I before I purchased a CallURL telephony board, I used a D-Link fax/data/voice modem DFM-565E. This modem connected with the serial port. There are other models which use USB. I don't know if D-Link is available in the US, but it works fine and is cheap. But not as good as the CallURL board I have now: reacts slower on key-presses; lower sound quality; didn't detect end-of-call tone on the phone line here, but if you're looking for a simple, cheap modem, it's OK. Further, see NCH's website for other modems.
Regards,
Martijn