It seems to be rare, it doesn't even happen as much as with 4leaf clovers. I have been searching for them for hours and found several. You can check them here https://steemit.com/life/@tarzanus/color-mutation-of-snowdrop-flower
I hope you find some nice snowdrops, good luck! :)
PS: I like them growing and I don't pick them. They perish way too fast if you take them into warm house anyway. :)