Just bought mine from a dealer near my home. Price is steep (~$20), but the last one was on the car for ~4 years and held up fairly well. Lexus part # 85242-0E030
Snap the plastic bolt cover off the rear wiper arm, then you'll be able to lift the blade all the way off the window. From there you can use a pen or screwdriver to release the plastic "catch" underneath the wiper arm attach point and slide the old wiper (and plastic housing) right off the arm. Works similar to the front wiper blade release mechanism, if you've changed those before.
Snap on the new blade and replace the bolt cover and you're all set.
Hope this helps!
