They are easy to get on valguero in the aberration cave. There is a lot of them spawning in the shallow rivers. Just clear out the spinos and crabs. Then just stand next to the river and tranq it. Just be careful. In some of the rivers there's a lot of electric eels also
Be really care when knocking these things out from a distance! Unfortunately they don't look knocked out when they are so continuing to shoot darts at them can be an easy an unfortunate mistake to make.