you have 3 options:
1) an external UART: high cost, simple to implement, high baud rate possible.
2) a "soft" UART: no cost, medium to implement, high baud rate not possible.
3) an external $0.50 uC such as an 87LPC:, low cost, complex to implement, high baud rate possible.

I am currently employing option 3, once you have it coded, you have extra UART(S) at a reasonable cost forever (the 87LPC will not change, the IIC code for your main controller is "standard").

Erik