Welcome to the club, jmb! As the nature of the beast, struts and shocks start wearing (just like brakes) from the day they are new. Many factors influence how quickly they wear; driving style, road conditions, etc. As others have mentioned, drifting, uneven tire wear, harsh reactions, and a general "unsettled" feel of the vehicle all indicate that the struts and shocks may need replacement.
Typically on my vehicles, I will replace shocks and struts at 60,000 miles as part of my OCD vehicle maintenance program.
As merling mentioned, you can buy the parts online from Infiniti. Another option is what I did on my previously owned 2006 M45: go with an aftermarket option. I chose KYB at that time. Both struts and both shocks were under $400 delivered. Having used KYB components before, I was confident in their quality, and they did not disappoint on the M45.