Link's Birthday - ran out of keys

[Hairball]

I'm up to Ganon's Castle in Link's Birthday but I'm out of keys.

What level is the magic key on? Or is there a shop where I can buy just one key?

In level 8 I wasted a key on a door I didn't need to open - I should have reset then and not done that, but it's too late now.

[Mr. Z]

Well, you can still go back to L8 to get the key.
You can get the magic key later in L9 at a moment you don't need it. So actually the key didn't have to be in the game.

[Hairball]

I mean that in Level 8 I got the keys but I used one on a door I didn't need to really open.

So now I have no keys and can't get any farther in level 9.

I must have missed one somewhere - but have no clue where I might have missed it. I'm guessing I need to go back to level 7 and find one - maybe. I hope so.