I would recommend taming a frog and taking it into the swamp cave on The Island. There's lots of dragonflys there which will give you thousands. Make sure to level health and melee on the frog as the creatures in that cave are high level and do a lot of damage, also bring a weapon like a shotgun for the larger insects that aren't weak to the frog. Lastly, remember that the dragonflys attacks drain your frogs stamina, so don't get cornered because you WILL die!