Online generally now. When I worked at a Barnes and Noble (in the cafe, I had coffee shop experience, they were hiring, it was near my husband's base), I had an employee discount and was constantly buying stuff there, but now it is generally easier to buy on Amazon, especially since I have their credit card and use the point system to get money off every so often. . . But that's in San Diego.
When back home
http://www.el.com/to/thedalles/ there are more familiar options. I provided that link so I could show that I'm not just making it up, my home town has my state's oldest bookstore. It's been operating continuously (though under different names from time to time) in the same location for over 130 years. It's a cute little building with a mix of new and used
books. I was in there at least once a week up until I got married and moved. I've also stumbled on some interesting out of print books in 1920's style binding in antique stores from time to time . . . and if you're ever in Oregon, go to Powells.
http://www.powells.com/info/places/burnsideinfo.html
I love bookstores and so does my husband. When I was working on Matt's days off, he would sometimes come in to B&N, get a coffee and sit in the cafe reading for a few hours . . . After I left there we would go back as customers and still do occasionally (Matt's got this thing about it being like a library so we can't go there if our baby is going to cry . . . I suppose he is right, but I have a high tolerance for other people's kids so I don't think that way. ).
I like online shopping too. I am constantly updating my amazon.com wish list just for the sake of categorizing all the books I want.
Edit: PS I've also purchased books at Library sales and garage sales, though at garage sales I generally find a lot of dime
novels, bodice rippers, and general trash and not a lot else. Off the top of my head I can only remember three books I've bought at garage sales other than children's picture books: a very worn copy of
The Hitchhikers Guide to the Galaxy which I read and then sent to my then boyfriend (now husband) in Iraq (it is probably still over there because I don't think he brought it back. Maybe a camel spider ate it);
The Hunt for Red October (I found two copies of this in one day at separate sales and bought both because I wanted to give one to my brother); and some random Hardy Boys
mystery . . .
The Witch's Key maybe?