Now if you'd asked me this a couple of weeks ago I'd have bitten your hand off - having only just completed collecting a full brigade of river trolls for my orc and goblin army. But I've got all I need now, sorry.
Having been slowly collecting and trading trolls with friends/on e bay for nearly a year now to acomplish this, my top tip is to buy more packs of trolls than you actually need, swap the river/stone trolls round until you have full units of them and then sell/swap them on. because you're offering all of 1 type, you can usually get at least what you paid for them, while at the same time hoovering up exactly the sort of trolls you need.
Dave
