Try MAXIM they do all maner of protocol converters or use a MAX232 + LTC485
this does 232 to TTL then TTL to 485 and not very expensive.

Another thaught We buy a PLC from Cuttler Hammer in the UK but its made by
Samsung and supplied with the software is a RS232 / RS485 converter in one of
those tiny boxes that takes just two 9 pin D type connecters about 25mm *
12mm not entirly sure what configuration it is but it does contain both of
the above chips and is line powered.

Cheers Steve.....