The two most popular USB to RS232 converters are based on either “CH340G” chip or the “PL2303” chip. Below are the drivers (latest as on 02-March-2016) for both.
While purchasing, check if the module has the DTR pin or not. It is not a necessity but will be easy to program the Arduino boards. Without the DTR line the Arduino will have to be RESET manually while uploading the codes to the Arduino.