Big rock (Hawthorne Hills in NE Seattle)

I am looking for a single, large-ish rock, (maximum) dimensions approximately 12" x 8" (with the third dimension relatively unimportant, except that somewhat thinner is better than thicker (preferably in N or NE Seattle, or close to those neighborhoods). This is for repairing a section of garden that had a small tree removed, which was helping to hold back the dirt...