Puppetswar has some pretty cool Orky stuff (buggys, a converted School bus, small and very big walkers) and "not Space Marines" as well as some tanks that fit a
SM/Guard/Scions. Also a new "not Tyranids" line
https://puppetswar.eu/
Models and Minis has orky stuff (Killa kanz with crab legs, half tracks and a "heavy tank" and pretty nice Guard vehicles (alternative missile launchers, Tanks, wheeled chimera and Taurox kits)
https://www.modelsandminis.com/
Blood and Skulls has lots of alternative weapons/turrets/sponsons/tracks/add-ons for tanks
https://www.ebay.de/str/bloodandskullsindustry
Anvil industry and Victoria Miniatures are my go to for Guard infantry. Victoria also has great artilery while Anvil also has Battlesisters and some
SM stuff.
https://www.anvilindustry.co.uk/
https://victoriaminiatures.com/
Kromlech was already mentioned. In their shop you should also find stuff from Spellcrow that is also very good.
Mortian has a growing line of tanks (trackes, wheeled, half tracked), including stuff for
IG Elysians and Space Skaven. Also crawlers including a Baneblade sized heavy crawler
https://mortian.shop/
Edit: and pretty obscure: Blightwheel miniatures has "Not Admech", and "not Guard", including pretty nice terror bird rough riders and cheaper Cyclops Demolition vehicles.
https://www.blightwheelminiatures.com/