Any trailer shop or spring shop will have them. You can cut them off any late model truck rear axle. Any of the speed shops should have them.
http://store.summitracing.com/egnsearch.asp?N=700+115+%2D126266&D=%2D126266
http://www.jegs.com/webapp/wcs/stores/servlet/KeywordSearchCmd?storeId=10001&catalogId=10002&langId=-1&N=0&Ntt=spring+perch&Ntk=all&Nty=1&D=spring+perch&Ntx=mode%2Bmatchallany&Dx=mode%2Bmatchallany&searchTerm=spring+perch