I'm 99% certain it can only be unlocked via gambling at the shrine in the Krypt (min. gamble - 500 koins.)

It might take you a while, though. I still haven't unlocked it.

The gamble is a simple system
Doing 500 non stop will give you nothing
500 will at most give you 3 to 5 items, so when you receive 250 koins back thats when you will increase bet by 1 lvl.
I know for the 1000s its 1 item per bet increase at most as in I never got anymore by re betting 5000 koins as oppose to going 5000, 5100,5200