These helper (or keeper) spring retainer seats are for 2.25″ ID springs and are available in two different sizes depending on the coil thickness of your spring.
The blue retainers fit springs with an OD of 3.295-3.440″.
The black retainers fit springs with an OD of 3.158-3.295″.
See chart in images for sizing for Hyperco springs (will work with any 2.25″ spring, just measure first.)
Hyperco helper springs can be found here.
Sold as a pair (two retainers).