I got my info from a DMM player. I'm not sure if it's been documented, at least in the JP wiki. Assuming they can be obtained from Jewel Gatcha, we should find out the draw rate from rerollers soon enough. I suspect we'll have two options to get one: spend thousands of dollars on Gatcha, or reroll. The draw rate should be too low for players to get them through normal play, or even by paying a reasonable "subscription" of $30/month.