Noely,
NI produce USB to Serial converters that use NI-VISA. The have either 1,2 or 4 ports and also come in RS232 and RS 485 flavours. You probably can buy third patry devices however I would recommend these due to the high level of integration with LV.
http://sine.ni.com/apps/we/nioc.vp?cid=12810〈=US
Hope this helps.
Kind Regards
Steven Bird
Applications Engineer
National Instruments
