One can purchase Bilstein shocks on their official website, or on third party distributors' websites such as All Shocks, Summit Racing or Off Road Warehouse.
The warranty you get with Bilstein shocks depends on where and how you purchased them. If they were fitted to a vehicle at the point of purchase then your warranty lies with the vehicle manufacturer and they would be your first port of call if you have a problem. If you have purchased your shocks separately then the terms will depend on the country in which you purchased them, but generally, if Bilstein determines that there is a defect with any of their shocks, they will replace or repair them.
Bilstein
Rear shocks are easy however the front shocks require spring compression devices that must be used properly or you could get seriously injured or killed. The Heavy duty Bilstein shocks are very popular with the people at our enthusiasts forum. www.trailvoy.com
Take a regular 4x4 Silverado, add skid plates, Bilstein shocks, and Z71 badging, and you have a Z71 Silverado.
If you replace the manual leveling shocks on the rear with regular shocks you also have to replace the rear coil springs with stiffer ones. Use gm part #15182559 or bilstein part #199021 to replace the coil springs. The coil springs that come with the manual leveling shocks are not stiff enough to use with regular shocks, causing the rear end to droop.
...You either have the auto-ride suspension, or like I do, the zw7 premium ride suspension w/Nivomat self-leveling shocks made by ZF-Sachs - look for the yellow tag with the "SACHS" name on it - by the way, about $500 each from GM, not including labor
The Z56 is the police package and includes bilstein shocks, v rated tires, special springs,big sway bars, Ultra HD cooling,certified speedo, etc.
Bilstein Monroe
There are many places where an individual can purchase FOX racing shocks. They are available for sale on Amazon and on eBay. Also, car part stores such as Kragen carry them.
A good place to find and purchase Rancho shocks would be some great online stores. Most importantly, many of the local automotive stores in local cities carry them also. One of the most famous that come to mind is Advanced Auto parts.
If I understand right, you want to take shocks from a Subaru and install them into a Nissan. Forget about it. Just purchase new shocks meant for the Nissan.
The Subaru WRX 22b is sometimes considered to be a Bilstein edition (22 for the 2.2litre engine, B for Bilstein) but the more accepted rationale for 22b is that it is "555" in hexidecimal, representing 555 cigarettes, a major Subaru rally sponsor at the time.