Like Lovinthe hd said, hard wiring the rears may be easier and cheaper that a wireless kit.
However, Best Buy sells the Rocketfish Wireless Speaker Kit (RF-WHTIB-A-C) for less than $100 and it works fine. (I installed one in my Mom's Condo.)
It's still required a power cable to the wall and two speaker cables from the rear receiver / amplifier; one to each speaker. The Parts Express package looks like a better deal if they have stock. You get some rear speakers at no cost and if they don't work well enough for you, then you can swap them out.
There are Bluetooth Speakers that eliminate one of the speaker wires as the receiver / amplifier is built into one of the speakers and the other speaker is simply hard wired from there. You would need a bluetooth transmitter hooked up to your Onkyo as well. This approach would typically cost more $$.