local bike swap meets or craigslist. You need to be a little more patient because they are not always readily available. But with patience you find great deals at a fraction of what ebay items sell for.
Example: For 2 months I have been looking for a Campy 8 speed derailleur to complete a build, I just bought one over the weekend at a local swap meet for $15 in 9/10 condition. The same derailleur would go for at least $70 on ebay.
Just this morning on the local craigslist I saw a front and rear 8 speed derailleur combo advertised for $45, a pretty good deal too.
good luck