RS-R products are now readily available in the US. Check out their website below.
http://www.rs-r.com/products.htm
I'm not sure if the JDM springs are different than the ones sold here, but the spring rates listed are more like the RS-R "race" springs than the RS-R "down". The spring rates are listed in the application guide on the website.
Phase2motortend.com (a very reputable merchant) has the "race" for $235 and the "down" for $225. The "race" seem like nice springs, but it's going to be tough to find a shock to match up with a spring rate that stiff.
Here's a recent discussion about the RS-R "race" springs...
http://www.nissaninfiniticlub....28731