The best way to find a large fish tank for low or no cost is to find a used aquarium that someone is trying to discard. This can take some time and patience, especially for larger tanks as they are in shorter supply; however, local classifieds, Craigslist and community websites dedicated to keeping fish may have them available. Expect to have to pick the tank up from the seller or to pay for delivery, which may become expensive.
Fish require more than just the tank itself to live. Most aquariums also need a hood, a water filter, a water heater, and a stand to keep the tank on. Since these parts need to properly fit the tank, look for someone selling or giving away the complete package.
When purchasing a tank used, ask for recent pictures taken in good lighting. If possible, pick it up in person because this provides a valuable opportunity to inspect the tank for cracks and other damage. Additionally, doing so avoids the cost of delivery.
Pet stores sell budget fish tanks new. Typically, these are smaller sizes and may be of poor quality. As such, it is usually preferable to find a higher-end, used tank rather than a new discount tank.