rs232 - Is MAX232 is needed? - Electrical Engineering Stack Exchange
USB to RS232 converter
UART Serial to RS-232 Breakout, MAX232, RS232 - ElectroDragon
Amazon.com: IIVVERR FT232RL USB to RS232 TTL Serial Adapter Max232 Module 3.3V 5V (FT232RL USB a RS232 TTL Adaptador serial Max232 Módulo 3.3V 5V : Everything Else
Alternatives of MAX232 in low budget projects - Do It Easy With ScienceProg