Feed stores. Call around. Ask for food grade DE. Some don't know the difference between that and industrial DE. Some don't carry it and some carry a 50# bag - which will last you a LONG time if you don't have many chickens. I have a 50# bag, which will last me forever. But since I can store it dry, I chose that lower cost option.
Also, before you do too much phoning around...check the forum posting under "Where am I? Where are you?" and see if there's a thread for BYC'ers in your state or area. If so, post your DE question there. Someone will know if there's a local place for food grade DE close to you. If not, you can always get it online - but you'll pay big for shipping because it's heavy.