Just as Dennis suggested, there are lots of options for serial card; National Instruments serial boards are guaranteed to be compatible with all National Instruments software --including LabVIEW-- as well as are other hardware such as your DAQ board. National Instruments has lots of resources for your serial needs and have a gateway dedicated just to serial
http://www.ni.com/serial. If you need more help selecting the appropriate serial board, feel free to contact your
Local Technical Representative and they will be happy to help you get the perfect hardware.
-Marshall R