Pretty much any health food store will stock it dried, as will any pan-Asian store with a Korean or Japanese section. I am reasonably certain I've seen it at Whole Foods, but you could call them to ask prior to driving.
Thanks! I'm definitely looking for dried hijiki, so I will check around. I figured that I might be able to find it at Asian grocery stores, but I didn't even think to look at Whole Foods. I appreciate the tips!