If you search ‘rain sensor’ on Amazon you can find plenty of options. The following is one that I’ve used in the past:
http://www.amazon.com/Orbit-57069-Sprinkler-System-Freeze/dp/B000A7SPPU
it’s also available at Home Depot and Lowes. Rain sensors are basically just a switch that’s activated by rain, and typically does not need power. So they work with both OpenSprinkler AC and DC versions.
The only complication is wireless rain sensors — a wireless rain sensor usually comes with a transmitter unit and a receiver unit, and the receiver unit is often powered by 24VAC (so that it can share the same power supply with the sprinkler controller). The receiver usually has 2 power wires, and 2 sensor wires. Because OpenSprinkler DC uses a 9VDC power supply, you will need to power the receiver separately with a 24VAC power supply. But you can still plug the sensor wires to OpenSprinkler DC’s rain sensor port.