I2C to UART bridge as breakout or in DIP package
Perhaps what you're looking for:

http://sandboxelectronics.com/?product=sc16is750-i2cspi-to-uart-bridge-module



From that page:




  SC16IS750 I2C/SPI to UART Bridge Module
  
  This module is designed to enable I2C/SPI to UART conversion. It works in I2C/SPI slave mode and interfaces with devices working in I2C/SPI master mode. There are also 4 extra GPIOs for IO expansion. NXP SC16IS750 is used as the key component on this module. Flow control and modem signals are also supported.
  
  This module is useful when extra UART interface is needed. Due to the multi-slave characteristics of I2C and SPI, many UART interfaces could be added to the system at the same time.