I noticed that we have never pointed out a Sirius-compatible receiver here on Operation Gadget. The Audiovox SIRPNP2 Shuttle Receiver is one of the models that's widely available. It looks fairly similar to the Delphi SKYFi Radio Receiver for XM and has similar features. The Audiovox Shuttle needs an additional Home Kit or Car Kit to work properly, another similarity with the SKYFi.
Sirius is obviously very competitive with XM, and this has translated into an intense rivalry in terms of design, manufacture, and sale of receivers. A lot of resellers offer discounts in co-operation with Sirius, so shop around and make the best deal that you can.