Agreed on $12-15. We paid our nanny $13/hour.
Also, if you do it legally, factor in money for taxes (unemployment and medicare/soc sec - usually you have to offer to pay both sides of the entitlement taxes).
You can look through an agency (most expensive), sitter city, northside parents network, word of mouth, craigs list (which i've heard the fewest good things about). THere are a LOT of nannies looking for work now, from what I hear, since a lot are losing jobs due to the economy.
If you decide to go this route, I have more specific advice - we've had nannies, and I've generally had a good experience, but have some thoughts now after three different nanny searches (we lost the first two due to personal issues that made them have to stop working, but i also learned things in both instances about what to look for in any future search).