The Innkeeper in White Orchard sells you cards that you can't get later. That's the only one I know for sure.
I believe I did buy up all her cards before she became unavailable. The NPC's only give a single card each right? I imagine it has to be set or else how would you get unique cards that can only be given by an NPC?
You can only get one card from beating NPCs, yes. They don't all give you specfic cards, though. Many of them are random.
Any tips for unarmed combat? having a hard time defeating enemies in hand to hand
Honestly, the best way I've found to beat enemies in hand to hand is just to dodge (O or B, not the X or A roll) to one side when they attack and then hit them back. You can get 2-5 hits in before they try to attack you again. I've beaten enemies 10+ levels higher than me without taking a single hit just using this technique. Trying to reposte in hand to hand with get your ass knocked out more often than you'd like.