The place to get the next Essence Stone is Ice Valley II in the El Nath region. Defeat White Fang and obtain Basic White Essence Stone. Make sure to fully prepare yourself since White Fang appears in cold regions.
#Talk to Eskalade by clicking on the light-bulb.
#Obtain 1 Basic White Essence Stone from White Fang.
#Talk to Eskalade again by clicking on the light-bulb.