Nice Price, Ed McCay's, Reader's Corner etc are great if you are looking for a book to actually READ, but as "filler", they aren't that cheap. You could get some off the outside rack for a dime, but most of those are really obsolete and dated, not to mention weathered by being outside. If the purpose is "decor", I don't think they'd do your shelves in your brand new house and justice.
Library book sales are a steal if you go the last day for the "$5 a box" rate, however, they will all have the library plastic on them as well as cataloging stickers, which are NOT easy to get off (says one who used to buy them to resell on ebay).
Thrift stores are indeed your bust buy--I particularly recommend ReTails off Old Wake Forest Rd. One Sunday a month, I think it's the last one (coming up), they have something like "half price book day" on books that are already cheap to start with, and the books they have are fairly current and in good shape. And, all of their proceeds go to spaying and neutering animals
You also might try Craigslist for an old encyclopedia set--looks good and takes up a lot of room!
(This is not a problem I would ever have because I have more books in my own house than I have room for!)