www.spencerlowracing.com has a couple of different length shackles you can purchase. Calmini has one as well, but I prefer the build of the SLR ones better. You can get urethane bushings for them if you want.
BTW, don't get shackles unless you have upgraded your stock leaf pack already with AAL's or new aftermarket leafs. Fixing the leafs first will give you a more satisfactory result.