I used to sell these typewriter when they were new and I never sold a Change-A-Type kit. I doubt many dealers ever stocked them and most buyers got them mail-order direct from Smith-Corona via the coupons that came with the typewriters. Nowdays your best bet is eBay where they come up from time to time. They sometimes are included with a S-C typewriter online or in local shops but it's not often. If you're in a university town your chances are much better as they were mostly an academic accessory. I once got the male/female slug when I bought a Smith-Corona from an ornithologist. Mostly you find Math and Greek symbols, often in sets of four. Have fun.

Uwe, I left a long, involved note about these on your blog at http://www.typewriterrentals.com/a-changeable-type/#comment-2587 .

Uwe, I would say at this point there were two keycap styles:
Rounded with at least three variations: early red, later black and white, and Sears-branded black, all these being interchangeable regarding fit;
Square with no variations but two applications, early square cream and square cream on plastic-bodied typewriters, these two being interchangeable and nearly indistinguishable;
And two slug types: a slip-on, and a slip-on with a catch, the latter being found only on plastic-bodied machines, but they are all interchangeable.
There are pics here: https://imgur.com/a/FVO802N . Feel free to use them if they are good enough.
I have only seen the square keytop with slug-with-a-catch on Sterling Automatics (all-electric with plastic body) and thus only the 1/4 over 1/2, but all the other square-top ones fit anyway, so....
Hope this is clear and helpful. Thanks for bringing it all together.

Don't know whether they were used on both manuals and electrics. Funny, I though the square keytops were pretty common but now I can't find any. I think I've mostly seen them on electrics. The whole keyboard has square keytops with only the barest space between them, like computer keyboards.
I think they were always called Changeable Type; don't recall where I got that "Change-A-Type" from; maybe it was Sears. I like "Change-A-Type" better.
I have added a Smith-Corona order form page to that Imgur album. It shows how seriously S-C took that keytop business.
Has anyone ever seen a Smith-Corona desktop or office typewriter with Changeable Type?? It seems like a natural and yet I have not seen one.
Whew. This is getting complicated.

The square keytops definitely look like they came from newer electric models, which would explain why I haven't come across them (I only have 6-series and older electric Smith-Coronas in my collection). You can see the Changeable Type used on those were the rounded keytops:


Clearly I'll have to update the page on Changeable Type to include the newer models. Of note, the order form that I have on the page also warns about selecting the correct keytops too when placing an order.