Check out well stocked art supply stores or hobby stores. Nationally, in the US, Hobby Lobby usually carries it and sometimes Michael's stores. But you'll find better prices online.
I'll buy 5 or 10 boxes at a time from Creative Wholesale .
They have the best price I've ever found. You'll pay around $12.00 or $13.00 in most stores, but only $8.00 to $9.00 here.
The only problem with SS Firm is if it has been sitting around too long it tends to dry out a little bit and get slightly crumbly. You can recondition the clay by kneading in a TINY amount of petroleum jelly or a TINY drop of mineral oil. Be very conservative in your use of these. You can always add a little more if you need too but too much will make it too gooey. If that happens add a little more dry crumbly clay. The use of a dedicated pasta machine will make this somewhat easier! Such machines are sold in hobby stores for use with polymer clays but you may pick one up cheaper at your local Walmart or other such store. Not mandatory, but if you are going to be conditioning a lot of clay it can be useful.
If I'm making a blade or spatula tool then that's the final step. But if I'm making a hook tool I then bend the steel into a hook shape then add serrations by dragging the edge of the tool repeatedly across the teeth of a flat file.
Whatever that means. But hey, Meatloaf is on it! I wonder what song of his will be used? 
I did not see that coming in tonight's episode! Wow!
