Sources for making trenchers as suggested by anons of the 4chan /tg/ /trench/ general:

The modular resin bits stores:

https://anvilindustry.co.uk/

https://madrobotminiatures.com/

https://victoriaminiatures.com/

Sources of conversion fodder for making your trenchers:

Bolt Action miniatures
https://store.warlordgames.com

Konflikt ‘47 miniatures
https://store.warlordgames.com

Frostgrave (cultists highly recommended)
https://www.northstarfigures.com

FireforgeGames
https://fireforge-games.com

VictrixMiniatures
https://www.victrixlimited.com/en-us

PerryMiniatures
https://www.perry-miniatures.com

GloomTrench (stls)
https://fickledice.com/gloom-trench/

Wargames Atlantic (plastic and stls)
https://wargamesatlantic.com