You can find the Armaspec Stealth Recoil Spring (SRS) Gen4 Kit (Carbine, H, H2) here.

Armaspec Stealth Recoil Spring (SRS) Gen4 is a self-contained multi-stage drop-in replacement for the buffer and spring. The Stealth Recoil Spring keeps the buffer spring from rubbing against the buffer tube, providing smooth, quiet operation while the second stage helps reduce the felt recoil significantly. The buffer mass is always under constant tension against the bolt carrier. Compatible with AR-15 with carbine-length buffer tube. Made in the U.S.A.

This Kit comes assembled as a Carbine buffer and includes an H and H2 weight so you can tune it the way you need it. Buffer Weight: 3.3oz (Carbine - installed), 6.6oz (H), 7.4oz (H2)
Length: 7.18"

The Stealth Recoil Spring is available in 7 variations with an equivalent buffer weight:
SRS-Carbine: 3.3oz
SRS-H: 3.8oz
SRS-H2: 4.7oz
SRS-H3: 5.6oz
SRS-BB: 5.3oz (For .458 SOCOM/.450 Bushmaster/.50 Beowulf)
SRS-9mm: 6.4oz.
SRS-308: 5.7oz

Actual Weight - Complete System (new version with stainless steel weight):
SRS-Carbine: 6.2oz
SRS-H: 6.6oz
SRS-H2: 7.4oz
SRS-H3: 8.0oz
SRS-BB: 7.5oz
SRS-9mm: 9.5oz.
SRS-308: 7.09oz