Onkyo uses infrared signals in the standard range supported by nearly all remote controls and infrared repeater/extender systems so you won't need anything specific to Onkyo.
You have a number of options:
1. RF (radio frequency) remote + base station. One-For-All 9910 on the cheaper end and MX-600/MX-800 on the higher end. These remotes convert infrared to RF and transmit the RF to the base station which turns it back into infrared. Line of sight from the base station to the components is best, but I've had good success with just placing the base station so that the infrared can bounce off of glass doors across from it. Some base stations include extra emitters on wires that you can place on or near the components, eliminating the need for line of sight from the base station to the components.
2. Infrared repeaters. Radio Shack sells them and a popular brand is Pyramid. Check out crutchfield.com to see a few of the brands that are available. These repeaters just repeat the infrared signal and do not convert to RF.
3. Infrared extenders. Same deal as the cheap repeaters except that they do convert to RF and relay the signal from the emitter to the receiver. They work with any remote control that uses standard ir frequencies (typically 44kHZ to around 100kHZ).
