supply crates are completely random if they spawn and what grade they are, there are no garentees for specific types of loot crates.

there is a cave on the northen-ish part of the volcano island in the east. i've never been in it as of yet.

there's a cave in north west part of the map on an island, that has what seams to be 5 loot drop areas i close proximity, but it's packed with Mantis'.

south of the swamp up the hills, but not all the way up is a long windy cave with 3 obvious loot spawn areas and maybe a few not so obvious locations (including a loot spawn in the ceiling..)