If your willing to wing it a little tanks are not a big problem.

One of the easiest, and most cost-effective, is to head down to the discount store and get a kiddie pool or two. A 6' diameter kiddie pool 12" deep holds about 200 gallons.

While there pick up some nylon scrim from the fabrics section and a large rubber ball. Drape the scrim over it and use the rubber ball to keep it above the water so bugs don't take up housekeeping.

I've seen someone mount kiddie pols rim-to-rim to make an nice tank. The key was arranging a piece of PVC pipe under both rims and screwing/bolting it together every few inches. I helped them bend the pipe. They used a urethane adhesive caulk after cleaning the plastic surfaces vigorously. Stacked you get about 400 gallons but joining struck me a lot of trouble to get the same capacity as two siting side-by-side.

Another alternative is to use 2bys joined at the ends with cheap galvanized steel angles to form a frame sitting on the ground. This can then be lined with sheep plastic to form a tank. If you can a safety liner intended for a waterbed you get pretty corners.

The main bladder for a waterbed will also work but bladders can be tricky. Sealed you avoid insects and contamination. Flexible they don't form vacuum. But cleaning and inspecting can be problematic.

Sheet polyethylene will work for simple open-frame tanks but exposure to sunlight degrades it. The black last longer in sunlight but all of it is pretty long lived if kept in the shade. It comes in various thicknesses. Thinner is cheaper but more prone to puncture.

Even simpler, but more labor, is to simply dig a hole and line it to create a pond. Most home centers will sell you a 'pond liner' which is heavy, typically black, PVC.

I would advise that if you try plastic lined tanks start with cheap Visqueen, sheet plastic, to assemble the project and get it all up and running. If you are still pleased with the design after a few months you can reline it with the more expensive liner materials.