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

A solid list of pen refills and compatible options.

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