If it's a 2007 or newer you can get up to two free nav discs.I know that you just bought your SRX.
Did you buy it from a Caddy dealer?
Is it CERTIFIED?
If it is CERTIFIED...I think you get one free update for the NAV...but you have to actually call the nav disk center.
Other than that...be patient and keep looking on ebay.
One will turn up shortly.
I was told by the dealer that the 2009 updates are on backorder. I am also not eligible for a free upgrade because my car is a 2006. Oh well...