Think you need to contact H&R directly - if they will supply half kits do not expect it to be cheap.
In the past I have contacted Eibach & AP for spares - both times they said yes.
But the prices they quoted was really high - eibach expected me to pay £60 for 2 Mk4 Golf Rear ARB bushes, AP thought £80 very reasonable for a single adjustable rear Lupo spring seat - both pretty much double what I was expecting.
It may be simpler to bite the bullet, get a full set replace them all and sell your old rear springs.