Where do you buy them ?
invite black lizard Xer to Bareshade's fair. He sells lots bombs.
Xer is to be found near Corrupted Lake in forgotten temple, when you sneak around there. During a day he's usually out to look for some ingredients, so that's the moment you can find him. Have a cleansing water, would be a good start.
But he's available after you complete good lizard's tribe ending.