I get most of mine at estate sales, but I'm looking for all kinds of bottles for resale. If looking for bottles for my collection I would have to go to 100 estate sales to find one bottle. Ebay has by far the best selection of bottles offered for sale. My wife uses etsy but the bottles on there are usually lower quality. There are auction houses for high end bottles that have online bidding. I seldom buy from those because they charge a buyers premium that increases your cost about 15%. Ebay charges the sellers 10% with no buyers premium.
These pages are always good for potential purchases although I have had very little luck selling things here. Go to the For Trade - Sale forum and shop around. Maybe you'll even buy one of mine.[8D]
Go to a bottle show in your area. There is a magazine called the Antique Bottle and Glass Collector. Go to their web site and check out their " show calendar ". That lists most all the shows in the country.
I have bought some bottles on Craigslist , but I do better selling on there . I did really well today selling things after posting some ads on Craigslist last night . Sold a good many bottles as well as gas and oil memorabilia . Doesn't always happen but the past couple of years I have done quite well . One fellow today was here last year in reference to an ad . Brought his brother with him this time and they both bought things . You just never know , it is like eBay or any other venue , if the right person is looking at the right time you have something they want , you will make a score .
Talk around town too. There might be someone around who also collects bottles that doesn't have the internet or whatever. I made a decent score today at a friends digging, though I didn't find any in the ground. Her mother came out and gave me all sorts of stuff. People like that are awesome. You never know unless you ask. Good luck!
I get a lot of my bottles at flea markets and antique shops. The only problem with flea markets is that it's hard to find really old bottles, when I go to one I usually get acls and crown top sodas. Though I have found some older stuff, I have never found anything pontilled at a flea market. But if you know what to look for, flea markets and antique shops can be good places to find nice bottles.