He's available if you did a good ending for lizard tribe. Take with you some of cleansing water. After you saved Roushk, go to the same corrupted lake and keep wandering around untill you find Xer. Here you need cleansing water actually.
Develop relationships with Xer, invite him on the fair, and there he alwats sales a lot of bombs
P.S. Personally I didn't use any bombs against slimes. I had enough health potions so I fought them. Were 5 of them or os, not a big deal.