Any custom mainspring made for SAA will fit the conversion. Wolff, Lee's, etc. And VTI Gunparts has a reduced power Pietta SAA spring that will fit. I do use an additional washer (like Coffinmaker suggested, but I use a stainless split washer instead of the leather) under my aftermarket mainspring to make it a tad lighter. If ever any failure to pop primers (I have not had that problem), then the washer can be removed to get a bit more power.