The My Little Pony Preservation Project recommends Swarovski, I'm not sure if you might have visited the website? http://www.mlppreservationproject.com/symbols.html#G2
One does also get glass crystals in similar sizes and various colours that do not sparkle as much as Swarovski. We get them in South Africa so I am sure you will be able to track them down. There are even plastic 'crystals', but I personally would not recommend them as they are pretty dull in comparison and might become scratched over time as well.
I hope that this helps, good luck!
I found the Preservation Project before I made an account here, actually, and that's because my Google-fu is in peak shape and is exercised daily.
Sites like that are not my go-to how-to, though. Even on that page alone when you scroll up to the Twinkle Eye replacements, there are links to places that are 5+ years old without any indication that you can still get those products from those individuals/places. That is frustrating when you're searching for information and turns me off from using web resources like that.
Thanks for the info, though. I do appreciate it.