Craigslist or something along those lines for used. Home Depot, Walmart, Sams Club, Lowes, Best Buy and just about every other store that sells home appliances for a new one. I use a 5 cubic for mine but about to upgrade here for a larger one since I'm starting to feed more raw food to my dogs.
Got it on sale at Home Depot for $130 last summer with free deliver. Google Deep Freezer or Chest Freezer and look for sales. I'm just waiting on the next decent sale to upgrade.