I found my 72 on collectorcartraderonline.com, Ebay works for some. If you were on the east coast, I would say the All ford nationals at carlisle, they have a huge car corral and there are usually a good number of classic stangs for sale.
you might also try the good guys show in pleasonton this weekend. they have an area for cars that are for sale and there's usually quite a few mustangs.
In my experience the best deals are not advertised. Let everyone know what you are looking for. Someone you know might know someone. Many years ago after searching, travelling and finding junk, someone I knew told me about a 68 she saw on her way to work with a for sale sign in the window. Got a solid vehicle at good price.
Also try putting a WANT ad on craigslist. State exactly what you are looking for and your price range. I did that when I was looking for a truck and got a good deal.
What is it that you're looking for? I've seen 2 65-66 coupes in a couple of fields and 1 66 coupe with a for sale sign in my neck of the woods. I've been told of a 68 coupe that a guy wants to basically give away for under $1000, not to mention I have contacts up in Southern Oregon who can keep an eye out for something like that. What is your price range?
Unfortunately, they are wherever you find them. A few years ago I started looking for a decent '67-'68 fastback and found the pool was dry. I looked on sites like www.collectorcartrader.com and found a few, one of which I traveled 12 hrs one way to buy, only to find a POS waithing for me. Then I found the ultimate find, a cherry, low mileage, one owner '68 fastback owned by the proverbial 93 year old lady sitting in a rest home parking lot with a dusty "For Sale" sign. The car had spent every nite in a garage since new up until she had to move into a rest home. The odd thing is, she'd offered the car to various relatives for free and not one of them wanted it! I snapped it up for $7200 and haven't been sorry since. Keep looking, be patient and don't buy a rusty car just to have Mustang and be prepared to pay more than you expected for the right car and you'll find one.
i found mine by word of mouth. let friends and familes know what your looking for. dont have to be specific but just say classic mustang or something. word of mouth spreads quick.