Recently purchased a 3pi+ 32U4 - just wondering what ultrasonic sensor would be recommended for this robot.?
You can find our selection of ultrasonic sensors here, all of which should be able to be read from a 3pi+. However, there are no free I/O pins on the 3pi+ by default, so you will probably need disable or disconnect one of the other features to add an ultrasonic sensor. This thread has some more discussion about what would be involved.
Also, in case you are not limited to just ultrasonic sensors, here is a link to all of our proximity sensors and range finders as well.