I believe you can buy Z32 lower mounts to thread on to your S13 coilovers from some coilover manufactures. I know splparts use to sell KTS coilovers with the option of buying Z32 lower mounts.

The sleeve/insert is a great way to build your suspension setup and tailor it to the way you want, however those kits are no cheaper than a mediocre set of coilovers. The inserts alone are 175-200 a piece, atleast the Koni's are...