Is there a reliable way of getting Armour piercing weapons?

I would take a good look at your liberation map, and try to liberate areas that unlock good weapons in the shop.

My shop now always offers a Armour Piercing Shortblade (the yellow rarity one) for 35 (credits?).

Some of the unlocks from liberations state "Make xxx Permanently available in the shop." Those are the ones you want to look for to make it easy.

That's the most reliable way I know of. If you're feeling a bit riskier and want an adventure, you can always go on your own looting adventures without taking a quest. Usually ships have junk; but every once in awhile they will have a lot of chests. I got one that had like 3-5 orange (rechargeable) items on it.