I think there may be an invisibility potion or two in the manor type houses in Beregost and/or Nashkel. There is one at least.
Maybe it's just my install, but the initial Dorn ambush bandits drop a bunch of invisibility potions. So make sure to talk to Dorn to trigger the ambush upon going to Nashkel mines.
As for invis detection, I always buy up the detection scrolls from High Hedge. There should be two?
I don't learn them typically, and keep them until the amazon ambush is done.
At the end of the day, the backstabbers in that ambush have a mere backstab multiplier of 2. You should be able to tank the backstabs relatively safely with any class able to wear a helmet, especially when not on double damage.
It's not even the lack of apr, it's just seeing that he cast harm means taking a brief time out until it expires.