For 3.3V there are special RS-232 drivers available.
Try:
ADM1385ARS
ADM3202ARU
LTC1386CS , or similar ..
or you can use any of MAX232 compatibile drivers but you will have to convert from 5V to 3.3V. For such a conversion you can try the attached circuit .. This is probably the cheapest option ..
Regards,
IanP