I’m looking forward to seeing what you do with these orcs, your goblins are excellent! I expect your painting style will also suit the sci-fi figures (old necromunda figs?).
I can’t help with sourcing the shields, but one possibility if you can’t find enough would be to use the blue heat-softened moulding plastic to make press-mounds of an unpainted shield. You can then use it to mould greenstuff replicas on a thin sheet of styrene backing and cut them out. They won’t have any details on the shield backs but folks won’t see that anyway when the models are assembled. Some won’t be as crisply moulded as the original, but with orcs that won’t be a problem- they aren’t known for their craftsmanship anyway!