Don't bother with the "big box" stores, it's the smaller, specialty lumber yards that are most likely to carry it. Be prepared for sticker shock, as the price of cedar has risen dramatically in the last year. The price of 4x4s around here (NH) ranges from $45 - $62 and of course, the cheapest dealer is 35 miles away. If you're doing this as a business, ask about wholesale/jobber discounts and you can also look for wholesalers in your area, though in my experience, they often only sell in pallet quantities.