Both Baco Ribbon and Fine Line offer black/red bichrome ribbon in most of these materials for an incredibly reasonable price:

• nylon ribbon $0.10 - $0.15/yard
• silk ribbon $0.33 to $0.40/yard
• cotton ribbon $0.25/yard

If you're going to buy even 3-6 spools of ribbon at individual prices of $9-20 per spool, you may as well make the investment in a half or full reel of inked typewriter ribbon and save yourself a lot of hassle. This will bring your spool of ribbon price down into the $2-4 range.

Ribbons Unlimited is great, but their prices on most ribbon is comparatively really high because part of what they're selling you is the information about which spools will fit your machine. This is fine if you get a typewriter without spools at all, but if you've got original spools, you can get really great ribbon for a fraction of the price and spool it onto your extant spools.

reply to https://old.reddit.com/r/typewriters/comments/1n58ivm/working_on_my_first_restoration_royal_arrow/

https://www.ribbonsunlimited.com/Inked-Nylon-Typewriter-Ribbon-Color-Swatches-s/12742.htm

The replacement typewriter ribbon for my Smith-Corona Clipper is 1/2" nylon ribbon with eyelets on 2" diameter spools.

https://www.ribbonsunlimited.com/category-s/5415.htm

Some cheap typewriter ribbon spools don't have grommets on them to force auto-reverse of the spool. Without grommets, some machines may sense ribbon tension for reversal, otherwise one needs to switch direction manually.

There are grommet installation tools that one can use, but this often requires getting one's hands dirty to install them.

Ribbons Unlimited has a good reputation in the r/typewriters community for providing good sales and service.