Hmm, maybe they only appear after the 50th or so Keese you've beaten? How many have you defeated? Fi tells you that when you Z-target one and press D-down. All youcan possibly do is run back and forth through the waterfall cave and beat many, many Keese.
EDIT: Ninja'd o.o
I'm pretty sure there's stuff you can upgrade with Monster Claws even at the beginning of the game, actually. Either way, this help thread was probably not made for something the OP didn't need help with. :bleh: And besides, it's good to stock up on the thing before you need it so you don't have to go out of your way to find it.Do you even need a monster claw when you're at the Skyview Temple? Only start worrying when you actually need the thing. Just go on a Keese massacre as you said, you should probably find one. Treasure medals help as well, you can get one from a goddess cube (not sure which one).