Where would I look to find the key to wind up the antique clock I've just bought at auction?

The clock has 3 holes in the face for winding but I can't find the key - on another old clock I have it tucks away in the case but I can't find anything on this clock. It also has a string underneath the case that when pulled makes the clock chime. It has 8 bells Westminster written on the top dial. How much would it cost to buy a replacement key if it is missing?