In Valguero I saw some of them on the bottom of a dragon nest along with Titanoboa's too and I believe Mantis too (don't remember too well) anyways I only found out because sadly ice dragons are gigantic jerks and froze me at the same time that my pteranodon ran out of stamina as I started to descend down to the lava I spotted the dung beetles before dying and losing my stuff. So if you have strong tames head to the snow zone and it should be around the edge of the top right side of your map once you see the ice dragons flying around there nest shouldn't be too far fly down and it should be there.
On the island, I think the easiest place to find them is in the lower south cave. Go in with a longneck or a shotgun, turn left at the first intersection, kill the boas and spiders I. The first chamber, near the end of it there’s usually 1-3 beetles. Tame, and carry back to base w/ Petra. Easy fertilizer.