I know there is an upgraded version of the beginning spell, but its not called Cure Wounds. Or at least the one I have isn't. I can't remember, and I don't feel like looking it up, but I think its "Strong Regeneration" or something like that.
Cure Wounds is a Man-at-arms skill. Strong Regenerate is the upgraded version of Minor Heal.
In reply to OP: I've seen it for sale, even towards the end of the game. But skill book availability seems very random, and for some reason the vendors seem to end up stocking some a lot more often than other throughout an entire playthrough. What you can do is to wait until you level up next, go to town and quicksave
before you speak to a vendor. Their inventory will change upon loading if they don't carry what you're looking for.