**repost from iwsti's "austinpike"**
There are a handful of pink springs. The most common ones people refer to as "JDM pinks" have a rate of 257 lbs/inch front, 217 lbs/inch rear, and are intended to match with the stock STi struts. SPT sells a "pink" spring that is sold through US dealers and is intended as an upgrade for the WRX. Rates are similar to the stock STi springs. Putting these on an STi will not be much of an upgrade. There are other JDM pinks (including the "Tuned by Arai" springs) that have higher rates than the standard "pinks" and are too stiff to be paired with the stock STi struts.
If you are buying new, your best bet is to buy from a vendor that is known to sell the JDM "pink" springs. Only a few places are importing them. These include: (list will be updated w/additional confirmed vendors)
If you are buying used, the way to determine whether you are getting the "JDM pinks" is by a paint marking on the second coil from the top - it should be light blue. The US/SPT pinks will have a white paint mark.
The Group-N strut top mounts are easier to come by as they can be imported by US Subaru dealers. A number of larger dealers have performance parts divisions that sell them. (As well as the vendors listed above.)
Buying used is more of a challenge, since the same mold is used to create both the stock and Group-N parts and they are essentially identical in appearance. If you can compare the parts directly to stock, you should be able to feel the harder rubber of the Group-N parts. Beyond that, the Gp-N parts carry a different paint marking.
stock front: white ------ gp-N front: green (+different location)
stock rear: pink --------- gp-N rear: white or yellow
In each pic the '06 mount is upper left, the GP-N is upper right, '05 mount is front center (uninstalled.) The '05 stock front has the paint pretty much rubbed off, but you can see a hint of it in the same position as on the '06.
Another pic of a completed rear assembly with Group-N top. This one appears to have a yellow paint marking, in a different location than stock.
(thanks to Rev for the image)
note that since this is iwsti.com, comments above apply to the '04-'06 Impreza WRX STi - I have no idea if any of the info carries over to parts intended for earlier cars or other models (Forester, etc.)
mods feel free to edit/update, others add comments/experiences. Especially feedback on the white/yellow rear marks (vendors?)