https://www.wellappointeddesk.com/2014/06/epic-refill-guide-rollerball-gel-and-ballpoints/

A solid list of pen refills and compatible options.

https://www.jetpens.com/Retro-51-Tornado-Classic-Lacquers-Rollerball-Pen-0.7-mm-Stainless-Body/pd/8233/refills

JetPens has a pretty comprehensive list of Parker style refills that will work with the Retro 51 Tornados, including ballpoints and a variety of nib sizes from broad to fine.

https://www.flickr.com/photos/rohdesign/5815595176/

Mike Rohde has previously had luck with the Bic Gel 0.7mm refill chopped down to fit his Retro 51 pens. Similarly he's done the Parker Gel 0.7mm refill and the Pentel Energel 0.7mm refill chopped down to fit.

https://www.wellappointeddesk.com/2015/10/ask-the-desk-retro-51-refills/

The Schmidt P8126 capless rollerball refill is compatible with the Retro 51 Tornado standard pens.

https://www.therefillguide.com/product-category/brand/retro1951/

In addition to their own branded rollerball refills, Retro 51 pens will also take a Parker style G2 ballpoint and capless rollerball refills.

Schmidt is another brand or compatible refills that is sold in Retro 51 packaging

https://www.leedsworldrefill.com/SRCH.html

This is the same as the Cross or C1 refill.

https://www.jetpens.com/blog/The-Ultimate-Guide-to-Pen-Refills/pt/231