The OEM receiver is expensive because they know they have a captive audience and the only place you can get it from dealers. Grubbs has always been one of the better deals around.
But any Sirius antenna with two connections, or one connection and a splitter, should work, so that does not have to come from the dealer.
If you can find one in a wreck, all you need is to buy an antenna and you should be in business.