I remember they were obtainable during Survival, but not remember how :D So, how? :D Thanks for answers!

According to SubetaLodge, they could originally be obtained from Skitters if you were a mature zombie. Looks like some sort of random gift/quest?

They can now be purchased at the Millionaire Center (in the winter section) for 500k. c:

They were also obtainable last year, it definitely wasnt only MC :D

I thought you meant originally. xD It looks like they were added to the MC in 2017 though - I don't think they've been released in any other manner recently.