Does LEGO sell a DC relay for Power Functions? I’d like to use a signal from a (5V) littleBits circuit to control some (9V) Power Functions motors.
Does LEGO sell or recommend a DC relay to facilitate this sort of thing?
If the answer is no, I’d welcome any ideas or experiences in how to do this in a way that’s both easy and LEGO-compatible.
LEGO doesn't make something to directly facilitate this type of integration as far as I know. If you can get a PF extension wire, you could cut it and directly make the connections to your driver circuit.

You can drive the motor using the middle wires of the PF cable (C1 and C2).
For what its worth, I would personally prefer an H-bridge such as this one over a relay as you get faster switching and can use PWM for speed control among other benefits. You'd power a board like that using a 9V supply and connect the C1 and C2 power lines to one of the outputs. Your 5V logic-level signals would connect to one set of inputs to control the motor.