Free reference images/sketches for building 3D characters? I think they're a
little difficult to come by, as only you know what kind of a character you have in your mind. And even if you do get free sketches, there might be copyright issues involved.
But still,
there's this site that offers human/skeleton reference images for sale.
Frankly, I'd suggest that you draw your own characters and use these to develop your model. All you essentially need is an outline, the main feature highlights and so on,
unless you're aiming for highly-detailed and extremely realistic models; but these aren't really suitable for game development anyway, are they?
For instance, this old [and simple] character of mine [below] was built by referring to a simple sketch that I drew myself.

[By the way, this post really belongs in the Graphic Design/3D section

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