I bought mine here:
http://www.grubbsperformance.com/Merchant2/merchant.mvc
I think Jamie at
http://www.nissanparts247.com/en_US/ will also give you a good price.
I used a Terk XM Micro on the roof through the third brake light.
If you have a center console shifter (not steering column shifter) I would use this mounting method:
Photos of simple XM install