C.R.
I have bought so many books from library book sales (usually sales are in the basement). I would just call up any library you are willing to travel to (or check websites).
The YMCA's once or twice a year have huge book sales (prices are better at the library sales though). Call them or check website.
You may also want to choose a local school to contact. The schools get money for selling Scholastic. They may let you purchase through them (you have to prepay, so I don't see that there would be problems...if your church has a school they probably do this and would let you contribute). If not try the nearest Catholic grade school.
I can't remember the last time I bought a book from a real store.