Office Depot and Staples may not carry ribbon. They don't in my area, and haven't for years. As far as eBay, the ribbon is generally universal, but the spool types vary. I often get spools that are of an awkward size and have to wind the ribbon onto different spools. I save spools I don't use in case I need them for other machines. On my machines I try to use the compatible/original metal spools if I have them.

I know the spool that looks like this, sold by a few eBay seller, fits an SM3:

You will see it advertised as being for different machines, but it is the same ribbon. Don't pay more than 6 bucks. There are others that fit as well, but this is the only one I know off the top of my head. As far as other colors, there are a few sellers on eBay that do different monochrome ribbon colors, and Baco ribbon can do both uncommon bi-chrome and monochrome ribbons. You have to email or call Baco, as they do not have a website.

Interesting that Staples doesn't carry ribbon where you are. I am able to find it consistently in my neck of the woods -- it's packaged as being for a printer, but it's the standard black ribbon (with grommets) on the universal spools. Porelon brand, #11579.

Definitely hang onto the original spools, especially if they are non-universal. You can always buy the ribbon and wind it onto the correct spools, though that may be a tedious and messy task.

GNUYORK, I'm interested to see if the metal spools/ribbon you purchased works in your SM3.  I have an SM3 and an SM4 that I just had the platens remade by JJ Short, and I'd like to finish both of the machines off with OEM metal spools and fresh ribbons.  If you find the ones you ordered work well, please let me know and I'll order up some from the ebay seller you suggested. 

Many thanks to all for the great info.  Much appreciated!

Thank you!  I'm placing my order today!