1) Check Goodwill's around 10 a.m. -- after they've unloaded their trucks. Bikes don't last long.
2) Hit the flea markets/swap shops EARLY.
3) Check out garage sales.
4) check behind fancy/schmancy bike shops for "throw-aways".
5) Put the word out to all friends that you'd like to buy an old 10 speed -- you'd be surprised what people will cough up out of garages/attics/basements/storage units.
6) be patient.