Or purchase a larger piece of wood and have it milled to size if you cannot do the work.

But avoid a 4x4 if it contains pith, as many do.

Or buy a premade window sill and cut to length. Lumber yards(not home improvement stores) have them.

+1 on handymac - lumberyards would carry 1 3/4" x 7" for a 2x4 wall and 1 3/4" x 9' for 2x6 walls and they are most generally made of Ponderosa Pine.

Thanks everyone. Will Do.