[GreenKeys] Source for 33 ribbons?

[Charles]

Where can I find ribbons for my Teletype 33's? Online searching is
getting me nowhere.

I'd read somewhere that Underwood typewriter ribbons were supposed to
work, but the generic ones at my local office supply store had spools
that did not match either the holes or the diameter. As an experiment
I bought one (1/2" x 10 yd, which is about right), unreeled it onto
the existing spools, and that works well except that there are no
"eyelets" to reverse direction, so I'll have to add those also. 

There must be an easier way. Aren't they manufactured any more?

thanks
Charles