I've had more luck with stones coming from the daily bags than from successful wild pet battles, but it took me a few days to actually find one from anywhere. I stopped "looking" after a while and forced myself to not bother/care. I found that this mentality made my experience less stressful. The stones are just a nice perk and very rare, so I've learned to not hope for one while pet battling.
Even so, don't stop trying. You'll find some eventually!