They are available on Rock Auto under "Suspension -> Shock Absorbers" The OEM magnaride shocks for the rear are $254 each which is not bad at all considering they are special. Replacing the rears was the best thing I ever did with my car. They are shot even if you do not think they are. The forces this car give on them really wrecks them. ~$500 makes your car feel like new. I also did the fronts but they were 360 a pop and it did not really do much for the handling (My struts were still good in the front after 95k miles)
The rears take the abuse and will typically begin failure by leaking. They will eventually lock up if you don't replace when leaking. I did three pair of rears in 208,000 miles on my 2010.
It is said that these can be replaced individually. For instance one is leaking the other is dry. I don't understand the finer points of how Magnarides work, but I figured the other would fail eventually anyway, so I replaced them in pairs.
I never replaced the front shocks. They did not leak and the ride was still the same. The Magnarides produce the best overall ride I have ever had in any car, period. Not to mention track time.
I have factory Magnarides on my 2011 GMC Yukon. I found this out when the air part of the rear shocks began to leak. The shocks were fine otherwise at 175,000 miles. I kept wondering why my shocks never needed replacing.
I have an updated set of Magnarides on my 2018 Z07. They are adjustable for driving conditions. There is an Eco mode (whatever this means), a Tour mode, a Sport mode, and a Track mode. I was driving mine in the sport mode. Five bent factory wheels later I switched to Tour. I should have been on Tour the entire time. The Tour mode is much more forgiving in the City. The City is hell on Sport. I still run Sport on the highway, along with my new forged wheels..