This is a wiki post, meaning all users can contribute. Please help make an extensive list of miniature companies (with links).
MANTIC GAMES
Really self explanatory: http://www.manticgames.com
Miniature manufacturers for Kingdoms of Men, League of Rhordia, and Brotherhood:
Warlord games (primarily ancients to vikings and the Italian wars (great landsknechts for LoR!)): https://store.warlordgames.com/collections/landsknecht https://store.warlordgames.com/collections/hail-caesar
Perry miniatures (100 years war and War of the Roses (perhaps the best miniature designers in the business)): https://www.perry-miniatures.com/
Fireforge games (ca. 9th to 14th century historical humans from Scandinavians to Byzantines, + fantasy humans (Great models that are easy to paint and convert; plastic Byzantines!)): https://fireforge-games.com/
Victrix (ancients and some dark ages (some of the best ancients miniatures around)): https://www.victrixlimited.com/collections/ancients
Gripping Beast (Dark Ages (Great late Romans, Vikings, and Arabs)): https://www.grippingbeast.co.uk/Gripping_Beast_Minatures.html
Footsore Miniatures (Vikings, historical): https://www.footsoreminiatures.co.uk/