Mkhand Industries (https://www.thingiverse.com/Mkhand_Industries/designs
) and Italianmoose (https://www.thingiverse.com/Italianmoose/designs
) are the sources I've had the most luck printing. Between them they've got a good selection of Imperials, Astartes, Chaos, Necrons, Tau, and Eldar.
Be wary of people uploading videogame file rips out of BFG
:Armada. They're really high-poly and not optimized in any way for 3d printing.