Are you looking for a slip clutch PTO to replace your current PTO or the add on clutch which uses your existing PTO (you may have to shorten your existing PTO to add on a clutch) Many tractor dealers carry the addon clutches, and I believe TSC and other "box stores" do as well.
If you have a standard round bore shear 1/2" bolt grade 2, you will need to ask for a Round bore to Round bore 1 3/8" shaft 2 plate clutch but you will need to use grade 8 bolts or you will still have a sear bolt system.
Check out <A target="_blank" HREF=http://www.agri-supply.com/catmain/3500005.htm>This Link</A> to Agri Supply. Look at item number 32323. $56 something plus shipping.
I got the TSC one for about $90. Yes! Shorten your PTO shaft. While a newbie, I had a bad break b/c I didn't shorten the shaft after installing it and tried raising the tiller a little farther than I didn't know was possible. <BANG!> I don't want to talk about it...