You can turn a cheap ultrasonic sensor (SR(F)04 or compatible) connected to an Arduino into a fully functioning I²C ultrasonic sensor.
To achieve this, I used an Arduino nano. The provided code should be adaptable for every available Arduino board.

The protocol for the communication is based on the one from the I2CXL-MaxSonar®-EZ™ Sensor Line of MaxBotix. Please note that I use an extended protocol!

You can find the source code for the Arduino board and the documentation on GitHub