Try KikkiJikki's Weapons Shops at Celeste Harbour or the Booth at Celeste Quarry. I'd be surprised if he didn't have what you are looking for.
You'll have trouble sourcing an ark-16 (L). It only has L bps and I am yet to loot one or see one turn up on auction.
I was looking for quite some time for this very weapon, but never found it. Not sure it actually still exists....
I’m not sure they do exist any longer??? I personally have not seen a bp ever only one I get is the Herman ASI 16 Artic.
I saw someone with a BP doing a price check on the forum here in the last 24 hours EDIT: In fact, it was Narfi in this thread and looks like Kikki is in possible negotiation to buy them atm, so Kiiki is still your best bet. Suggest you keep contact with him
So I managed to pick up an L bp with one click on it but unfortunately the craft attempt was unsuccessful. Sorry.