In all honesty I think eldar gaurdiens would be your best bet. almost all eldar have the same army patterns and rather similar helmets. all you would need to find is some backpacks for the warpspider and some head plumes for the dark reapers. first thing that comes to mind is chaos space marines and orks for the plumes, both of which you may find on bitz box websites, or even asking around in the trading area.
As for the backpacks im not to sure, they are quite a unique shape. I would consider buying the autarch which comes with a separate warpspider backpack then buying some "instant mould" and casting up all the backpacks.