Unbelievably terrific
I use Amazon, and occasionally Half.com. Another good source is Alibris.com.
Locally I go to a weird place called The New England Mobile Book Store in Newton, MA. If it ever was mobile in any way, it certainly isn't now. It's housed in the most rickety fire-trap looking building I ever saw, and even though I've been there often and often, there are still nooks/corners/crannies/niches that I haven't yet visited - probably some I don't even know about in the first place. And oh, boy, is the staff ever au courant about the stock!
It has everything, all shelved by publisher (which can get pretty confusing, since publishers are constantly merging and renaming themselves), but fortunately they put updated location lists at the end of every row of shelves. It is, however, definitely possible to get totally lost there. I always feel I should take my lunch, or a ball of thread a la Ariadne and Theseus, just in case.
BTW, New England Mobile isn't a secondhand/rare/OP place. For those things I go to Amazon, Half.com, Alibris, or locally, The Brattle Book Shop in downtown Boston (www.brattlebooksshop.com).
I was brought up in Manhattan and one of our regular - like weekly - treats was to go the a nearby place, the Lenox Hill Book Shop (long gone, snff snff). We also had a chain called Womrath's, and of course the
real Brentano's - not that thin chain that calls itself Brentano's today.
Finally, there were all the secondhand bookstores down on Fourth Avenue, and also on 59th St. Lenox Hill was for weekdays, but Fourth Avenue, and the stores on 59th St were special, for Saturday afternoons when my father didn't have to work.