I've got a blue one of these. Great typewriter. I order my ribbons here: https://www.amazon.com/Smith-Corona-17657-Ribbon-Typewriters/dp/B00006RVRZ/ref=sr_1_11?crid=3GRLFGDKLVB69&keywords=smith+corona+typewriter+ribbon&qid=1573170854&sprefix=smith+coronoa%2Caps%2C184&sr=8-11
The coronamatic 17657 is the Black Nylon ribbon and is either still in production or of very recent manufacturer. Amazon listing with correct picture is here: https://www.amazon.com/Smith-Corona-17657-Ribbon-Typewriters/dp/B00006RVRZ/ref=pd_sim_236_2?ie=UTF8&dpID=41ASCQE7TJL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&psc=1&refRID=J2MJF5KB6JBTRMJ395V2 (This is the one I have for mine)
Selectrics, with the exception of the the Model 72 (Selectric I Fabric ribbon) only have third party film ribbons available new, the Model 72 which used the fabric ribbon cartridge has nylon ribbons available for it.
Amazon Examples (none are actual IBM, but third party):
Model 71 (film spools): https://www.amazon.com/IBM-Selectric-Typewriter-Ribbon-Compatible/dp/B001D4F9E4/ref=sr_1_12?ie=UTF8&qid=1467922700&sr=8-12&keywords=selectric+ribbon
Model 72 (Fabric Cartridge): https://www.amazon.com/Typewriter-Ribbon-Selectric-General-Fabric/dp/B000HRXI1E/ref=sr_1_18?ie=UTF8&qid=1467922700&sr=8-18-spons&keywords=selectric+ribbon&psc=1
Well. I'm just going to answer my own question. Found the proper replacement ribbon on Amazon here .
I had come up with this link before, but I was a little paranoid that it wouldn't be quite the correct ribbon, but some digging showed that it was.