Was looking through a few options for good proxies (some of which I have just from collecting) for C3V/SoV stuff.
The Massive Darkness Troglodytes box has about 6 squads of dreadgul proxies, a minotaur that can just barely fit on a 40mm base, and some hero figures that could be used for martial la hire and Myrddin or just another squad of dreadguls.
A mind drinker vampire from D&D that could be used for Nicholas Esenwien. Much cheaper at $3 instead of the $15 or so the figure currently goes for.
A Cyclops from D&D that can be used for Bramcephys or the other cyclops that's in the process.
https://imgur.com/s1zZdNW
Some orcs from the Nolzur's Marvelous Miniature D&D line that work well as brute gruts, there are painted orcs in the pre-painted D&D line but are expensive being like $7 per orc. These ones are around $5 for two.
A couple of options for Kozil. Ones a reaper bones owlbear and the other is a Wizkids owlbear. There are also some more options like the Reaper bones black owlbear (not on their website anymore, but will be back eventually and is on the secondary market) and the owlbear that comes with the D&D Nolzur's paint set (a decent set of paints to start out with, though a bit expensive if not bought with a discount).