The Beauty Brains suggest using the ingredients list of products like mascara to find dupes (step-by-step instructions on their page, using Benefit's They're Real as an example). I've done similar things with skincare products, but admittedly I've never tried it with makeup. It does seem like it would work better with a product like mascara than, say, lipstick or eyeshadow, where there would be more factors to take into account besides ingredients.
Have you searched for dupes this way? Does it work?