[GreenKeys] Teletype ribbon 24704?

[Charles]

I am looking for a source of these excellent ribbons for my
ASR-33's... I only have two (now, one) left and have no idea where I
bought them. Despite being in the box for at least 6-7 years (and of
unknown age before that), it is printing perfectly legibly and the
purplish ink contrasts nicely on my canary-yellow roll paper.

Yes, there are plenty of sources for Underwood typewriter ribbons but
I have several of the nylon ones and the ink dries out in storage.
Since I don't use my Teletypes often, this is a problem.

They came in a plain white box labeled "UPI Teletype 14-15, Purple
Cotton Teletype 24704 1/2" x 10 yds". Only one spool, but that's not a
problem. Appears to be cloth (cotton!), not nylon.

ebay and Amazon aren't coming up with anything... any ideas?

thanks
Charles