Take the one bolt you have for the WP pulley and go to a hardware store and they will be able to match it with the same thread/pitch/length you need.
If I remember they're 8mm bolts on the WP pulley.
Even if the previous owner didn't install all of the bolts it still should turn. Can you turn the WP pulley without the belt on it? I bet it seized up. Time for a new water pump.