I'd kit-bash from plastics. You could grab the legs and head from any 18th-cent. plastic (Perry, WGF) model, and toss on the shirtless upper body of a Celt (probably Warlord). Clip hands/weapons and add the appropriate gear, there you go.
Granted, this requires sprues from several sets, so maybe not practical, but if you're invested on a couple of periods it could work. You could always take the jacketed upper body that you cut off if those legs and use it for a conversion of a woodland Native American wearing a European coat...
(you could also do the exact same thing with metal minis of your choice).