Those are the most desired hours, so it can be difficult.
Do you know any SAHMs? If so, you could potentially work out a fantastic deal. SAHM gets the nanny for the mornings so she can run errands, work out, etc. Then you get the nanny for the time slot you want.
And, with that, there's the flexibility for snow days or sick days. Providing the SAHM is flexible on those things.
College students could work, but some are unreliable, and you run the risk of their class schedule changing from semester to semester.