Epazote dies down during the winter, but the spring crop is ready to start, and fresh epazote can be found now in some markets that specialize in Mexican ingredients. It will be more abundant later in the season.
Hoja santa (acuyo) is a large-leafed plant almost impossible to find fresh. Latino markets may carry crumbled dried hoja santa, sometimes under the name hierba santa. The flavor is not the same, but it may be used as a substitute.