what size is the end?
If it's a 3/4" square you can rip them from 3/4" Cedar boards from the lumber yard or a Home Depot. They will tend to curve every which way when you cut them ...at least in my experience. You are building the "log cabin" bird houses...right? Those also show the corners are rounded off a bit, a nice job for a router table with a round over bit.
If it's at all possible to make them yourself, that's what I would do. To purchase them that way may prove quite expensive because of the number of passes to get each stick cut and rounded off....5, if my guess is correct.
I would also start with 1 X 10's about 3 or 4 ft long. Much longer than that and I think they will curve too much, but you could try.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)