Atlas Quest and the "official" Letterboxing site are the two ways that one can find locations of letterboxes I also have discovered an app that I use on my phone, but not all the boxes are found on all the sites.
I'm hoping this will be my ongoing list of letterboxing adventures.
Girl Scouts Rock! (LbNA) COULD NOT FIND (LaTorte Spring Trail, nr. Carlisle, PA) with Ella 4/8/15
2. Gloria's Pepsi (AQ) COULD NOT FIND (King's Gap, nr. Carlisle, PA) with Ella 4/11/15
3. Beautiful Spring (AQ) FOUND! (I-81 N Rest Stop just south of Carlisle, PA) with Ella 4/11/15
4. Irish Bell (AQ) (LbNA) FOUND (Michael Perry Park, Tucson) 4/19/15
5. Daisy Gordon: GS Series (AQ) COULD NOT FIND (Quincie Douglas Library, Tucson) 5/10/15
6. This One's For the Kids (LbNA) COULD NOT FIND (Children's Memorial Park, Tucson) 5/10/15
Tip 1: If a box has not been found in a number of years, even though it's still "active," it might not be worth the time and effort you put into trying to find it - unless your stroll/walk/hike is the main reason for your adventure.
Tip 2: I know that I'll be adding more and more as I become a more experienced letterboxer.